RFC 7540 – Hypertext Transfer Protocol Version 2 �HTTP/2�
Internet Engineering Task Force (IETF) M. Belshe
Request for Comments: 7540 BitGo
Category: Standards Track R. Peon
ISSN: 2070-1721 Google, Inc
M. Thomson, Ed.
Mozilla
May 2015
Hypertext Transfer Protocol Version 2 (HTTP/2)
Abstract
This specification describes an optimized expression of the semantics
of the Hypertext Transfer Protocol (HTTP), referred to as HTTP
version 2 (HTTP/2). HTTP/2 enables a more efficient use of network
resources and a reduced perception of latency by introducing header
field compression and allowing multiple concurrent exchanges on the
same connection. It also introduces unsolicited push of
representations from servers to clients.
This specification is an alternative to, but does not obsolete, the
HTTP/1.1 message syntax. HTTP’s existing semantics remain unchanged.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7540.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust’s Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction …………………………………………….4
2. HTTP/2 Protocol Overview ………………………………….5
2.1. Document Organization ………………………………..6
2.2. Conventions and Terminology …………………………..6
3. Starting HTTP/2 ………………………………………….7
3.1. HTTP/2 Version Identification …………………………8
3.2. Starting HTTP/2 for “http” URIs ……………………….8
3.2.1. HTTP2-Settings Header Field …………………….9
3.3. Starting HTTP/2 for “https” URIs ……………………..10
3.4. Starting HTTP/2 with Prior Knowledge ………………….10
3.5. HTTP/2 Connection Preface ……………………………11
4. HTTP Frames …………………………………………….12
4.1. Frame Format ……………………………………….12
4.2. Frame Size …………………………………………13
4.3. Header Compression and Decompression ………………….14
5. Streams and Multiplexing …………………………………15
5.1. Stream States ………………………………………16
5.1.1. Stream Identifiers ……………………………21
5.1.2. Stream Concurrency ……………………………22
5.2. Flow Control ……………………………………….22
5.2.1. Flow-Control Principles ……………………….23
5.2.2. Appropriate Use of Flow Control ………………..24
5.3. Stream Priority …………………………………….24
5.3.1. Stream Dependencies …………………………..25
5.3.2. Dependency Weighting ………………………….26
5.3.3. Reprioritization ……………………………..26
5.3.4. Prioritization State Management ………………..27
5.3.5. Default Priorities ……………………………28
5.4. Error Handling ……………………………………..28
5.4.1. Connection Error Handling ……………………..29
5.4.2. Stream Error Handling …………………………29
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5.4.3. Connection Termination ………………………..30
5.5. Extending HTTP/2 ……………………………………30
6. Frame Definitions ……………………………………….31
6.1. DATA ………………………………………………31
6.2. HEADERS ……………………………………………32
6.3. PRIORITY …………………………………………..34
6.4. RST_STREAM …………………………………………36
6.5. SETTINGS …………………………………………..36
6.5.1. SETTINGS Format ………………………………38
6.5.2. Defined SETTINGS Parameters ……………………38
6.5.3. Settings Synchronization ………………………39
6.6. PUSH_PROMISE ……………………………………….40
6.7. PING ………………………………………………42
6.8. GOAWAY …………………………………………….43
6.9. WINDOW_UPDATE ………………………………………46
6.9.1. The Flow-Control Window ……………………….47
6.9.2. Initial Flow-Control Window Size ……………….48
6.9.3. Reducing the Stream Window Size ………………..49
6.10. CONTINUATION ………………………………………49
7. Error Codes …………………………………………….50
8. HTTP Message Exchanges …………………………………..51
8.1. HTTP Request/Response Exchange ……………………….52
8.1.1. Upgrading from HTTP/2 …………………………53
8.1.2. HTTP Header Fields ……………………………53
8.1.3. Examples …………………………………….57
8.1.4. Request Reliability Mechanisms in HTTP/2 ………..60
8.2. Server Push ………………………………………..60
8.2.1. Push Requests ………………………………..61
8.2.2. Push Responses ……………………………….63
8.3. The CONNECT Method ………………………………….64
9. Additional HTTP Requirements/Considerations ………………..65
9.1. Connection Management ……………………………….65
9.1.1. Connection Reuse ……………………………..66
9.1.2. The 421 (Misdirected Request) Status Code ……….66
9.2. Use of TLS Features …………………………………67
9.2.1. TLS 1.2 Features ……………………………..67
9.2.2. TLS 1.2 Cipher Suites …………………………68
10. Security Considerations …………………………………69
10.1. Server Authority …………………………………..69
10.2. Cross-Protocol Attacks ……………………………..69
10.3. Intermediary Encapsulation Attacks …………………..70
10.4. Cacheability of Pushed Responses …………………….70
10.5. Denial-of-Service Considerations …………………….70
10.5.1. Limits on Header Block Size …………………..71
10.5.2. CONNECT Issues ………………………………72
10.6. Use of Compression …………………………………72
10.7. Use of Padding …………………………………….73
10.8. Privacy Considerations ……………………………..73
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11. IANA Considerations …………………………………….74
11.1. Registration of HTTP/2 Identification Strings …………74
11.2. Frame Type Registry ………………………………..75
11.3. Settings Registry ………………………………….75
11.4. Error Code Registry ………………………………..76
11.5. HTTP2-Settings Header Field Registration ……………..77
11.6. PRI Method Registration …………………………….78
11.7. The 421 (Misdirected Request) HTTP Status Code ………..78
11.8. The h2c Upgrade Token ………………………………78
12. References …………………………………………….79
12.1. Normative References ……………………………….79
12.2. Informative References ……………………………..81
Appendix A. TLS 1.2 Cipher Suite Black List …………………..83
Acknowledgements …………………………………………..95
Authors’ Addresses …………………………………………96
1. Introduction
The Hypertext Transfer Protocol (HTTP) is a wildly successful
protocol. However, the way HTTP/1.1 uses the underlying transport
([RFC7230], Section 6) has several characteristics that have a
negative overall effect on application performance today.
In particular, HTTP/1.0 allowed only one request to be outstanding at
a time on a given TCP connection. HTTP/1.1 added request pipelining,
but this only partially addressed request concurrency and still
suffers from head-of-line blocking. Therefore, HTTP/1.0 and HTTP/1.1
clients that need to make many requests use multiple connections to a
server in order to achieve concurrency and thereby reduce latency.
Furthermore, HTTP header fields are often repetitive and verbose,
causing unnecessary network traffic as well as causing the initial
TCP [TCP] congestion window to quickly fill. This can result in
excessive latency when multiple requests are made on a new TCP
connection.
HTTP/2 addresses these issues by defining an optimized mapping of
HTTP’s semantics to an underlying connection. Specifically, it
allows interleaving of request and response messages on the same
connection and uses an efficient coding for HTTP header fields. It
also allows prioritization of requests, letting more important
requests complete more quickly, further improving performance.
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The resulting protocol is more friendly to the network because fewer
TCP connections can be used in comparison to HTTP/1.x. This means
less competition with other flows and longer-lived connections, which
in turn lead to better utilization of available network capacity.
Finally, HTTP/2 also enables more efficient processing of messages
through use of binary message framing.
2. HTTP/2 Protocol Overview
HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2
supports all of the core features of HTTP/1.1 but aims to be more
efficient in several ways.
The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each
frame type serves a different purpose. For example, HEADERS and DATA
frames form the basis of HTTP requests and responses (Section 8.1);
other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are
used in support of other HTTP/2 features.
Multiplexing of requests is achieved by having each HTTP request/
response exchange associated with its own stream (Section 5).
Streams are largely independent of each other, so a blocked or
stalled request or response does not prevent progress on other
streams.
Flow control and prioritization ensure that it is possible to
efficiently use multiplexed streams. Flow control (Section 5.2)
helps to ensure that only data that can be used by a receiver is
transmitted. Prioritization (Section 5.3) ensures that limited
resources can be directed to the most important streams first.
HTTP/2 adds a new interaction mode whereby a server can push
responses to a client (Section 8.2). Server push allows a server to
speculatively send data to a client that the server anticipates the
client will need, trading off some network usage against a potential
latency gain. The server does this by synthesizing a request, which
it sends as a PUSH_PROMISE frame. The server is then able to send a
response to the synthetic request on a separate stream.
Because HTTP header fields used in a connection can contain large
amounts of redundant data, frames that contain them are compressed
(Section 4.3). This has especially advantageous impact upon request
sizes in the common case, allowing many requests to be compressed
into one packet.
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2.1. Document Organization
The HTTP/2 specification is split into four parts:
o Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is
initiated.
o The frame (Section 4) and stream (Section 5) layers describe the
way HTTP/2 frames are structured and formed into multiplexed
streams.
o Frame (Section 6) and error (Section 7) definitions include
details of the frame and error types used in HTTP/2.
o HTTP mappings (Section 8) and additional requirements (Section 9)
describe how HTTP semantics are expressed using frames and
streams.
While some of the frame and stream layer concepts are isolated from
HTTP, this specification does not define a completely generic frame
layer. The frame and stream layers are tailored to the needs of the
HTTP protocol and server push.
2.2. Conventions and Terminology
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this
document are to be interpreted as described in RFC 2119 [RFC2119].
All numeric values are in network byte order. Values are unsigned
unless otherwise indicated. Literal values are provided in decimal
or hexadecimal as appropriate. Hexadecimal literals are prefixed
with “0x” to distinguish them from decimal literals.
The following terms are used:
client: The endpoint that initiates an HTTP/2 connection. Clients
send HTTP requests and receive HTTP responses.
connection: A transport-layer connection between two endpoints.
connection error: An error that affects the entire HTTP/2
connection.
endpoint: Either the client or server of the connection.
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frame: The smallest unit of communication within an HTTP/2
connection, consisting of a header and a variable-length sequence
of octets structured according to the frame type.
peer: An endpoint. When discussing a particular endpoint, “peer”
refers to the endpoint that is remote to the primary subject of
discussion.
receiver: An endpoint that is receiving frames.
sender: An endpoint that is transmitting frames.
server: The endpoint that accepts an HTTP/2 connection. Servers
receive HTTP requests and send HTTP responses.
stream: A bidirectional flow of frames within the HTTP/2 connection.
stream error: An error on the individual HTTP/2 stream.
Finally, the terms “gateway”, “intermediary”, “proxy”, and “tunnel”
are defined in Section 2.3 of [RFC7230]. Intermediaries act as both
client and server at different times.
The term “payload body” is defined in Section 3.3 of [RFC7230].
3. Starting HTTP/2
An HTTP/2 connection is an application-layer protocol running on top
of a TCP connection ([TCP]). The client is the TCP connection
initiator.
HTTP/2 uses the same “http” and “https” URI schemes used by HTTP/1.1.
HTTP/2 shares the same default port numbers: 80 for “http” URIs and
443 for “https” URIs. As a result, implementations processing
requests for target resource URIs like “http://example.org/foo” or
“https://example.com/bar” are required to first discover whether the
upstream server (the immediate peer to which the client wishes to
establish a connection) supports HTTP/2.
The means by which support for HTTP/2 is determined is different for
“http” and “https” URIs. Discovery for “http” URIs is described in
Section 3.2. Discovery for “https” URIs is described in Section 3.3.
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3.1. HTTP/2 Version Identification
The protocol defined in this document has two identifiers.
o The string “h2” identifies the protocol where HTTP/2 uses
Transport Layer Security (TLS) [TLS12]. This identifier is used
in the TLS application-layer protocol negotiation (ALPN) extension
[TLS-ALPN] field and in any place where HTTP/2 over TLS is
identified.
The “h2” string is serialized into an ALPN protocol identifier as
the two-octet sequence: 0x68, 0x32.
o The string “h2c” identifies the protocol where HTTP/2 is run over
cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade
header field and in any place where HTTP/2 over TCP is identified.
The “h2c” string is reserved from the ALPN identifier space but
describes a protocol that does not use TLS.
Negotiating “h2” or “h2c” implies the use of the transport, security,
framing, and message semantics described in this document.
3.2. Starting HTTP/2 for “http” URIs
A client that makes a request for an “http” URI without prior
knowledge about support for HTTP/2 on the next hop uses the HTTP
Upgrade mechanism (Section 6.7 of [RFC7230]). The client does so by
making an HTTP/1.1 request that includes an Upgrade header field with
the “h2c” token. Such an HTTP/1.1 request MUST include exactly one
HTTP2-Settings (Section 3.2.1) header field.
For example:
GET / HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings:
Requests that contain a payload body MUST be sent in their entirety
before the client can send HTTP/2 frames. This means that a large
request can block the use of the connection until it is completely
sent.
If concurrency of an initial request with subsequent requests is
important, an OPTIONS request can be used to perform the upgrade to
HTTP/2, at the cost of an additional round trip.
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A server that does not support HTTP/2 can respond to the request as
though the Upgrade header field were absent:
HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html
…
A server MUST ignore an “h2” token in an Upgrade header field.
Presence of a token with “h2” implies HTTP/2 over TLS, which is
instead negotiated as described in Section 3.3.
A server that supports HTTP/2 accepts the upgrade with a 101
(Switching Protocols) response. After the empty line that terminates
the 101 response, the server can begin sending HTTP/2 frames. These
frames MUST include a response to the request that initiated the
upgrade.
For example:
HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c
[ HTTP/2 connection …
The first HTTP/2 frame sent by the server MUST be a server connection
preface (Section 3.5) consisting of a SETTINGS frame (Section 6.5).
Upon receiving the 101 response, the client MUST send a connection
preface (Section 3.5), which includes a SETTINGS frame.
The HTTP/1.1 request that is sent prior to upgrade is assigned a
stream identifier of 1 (see Section 5.1.1) with default priority
values (Section 5.3.5). Stream 1 is implicitly “half-closed” from
the client toward the server (see Section 5.1), since the request is
completed as an HTTP/1.1 request. After commencing the HTTP/2
connection, stream 1 is used for the response.
3.2.1. HTTP2-Settings Header Field
A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly
one “HTTP2-Settings” header field. The HTTP2-Settings header field
is a connection-specific header field that includes parameters that
govern the HTTP/2 connection, provided in anticipation of the server
accepting the request to upgrade.
HTTP2-Settings = token68
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A server MUST NOT upgrade the connection to HTTP/2 if this header
field is not present or if more than one is present. A server MUST
NOT send this header field.
The content of the HTTP2-Settings header field is the payload of a
SETTINGS frame (Section 6.5), encoded as a base64url string (that is,
the URL- and filename-safe Base64 encoding described in Section 5 of
[RFC4648], with any trailing ’=’ characters omitted). The ABNF
[RFC5234] production for “token68” is defined in Section 2.1 of
[RFC7235].
Since the upgrade is only intended to apply to the immediate
connection, a client sending the HTTP2-Settings header field MUST
also send “HTTP2-Settings” as a connection option in the Connection
header field to prevent it from being forwarded (see Section 6.1 of
[RFC7230]).
A server decodes and interprets these values as it would any other
SETTINGS frame. Explicit acknowledgement of these settings
(Section 6.5.3) is not necessary, since a 101 response serves as
implicit acknowledgement. Providing these values in the upgrade
request gives a client an opportunity to provide parameters prior to
receiving any frames from the server.
3.3. Starting HTTP/2 for “https” URIs
A client that makes a request to an “https” URI uses TLS [TLS12] with
the application-layer protocol negotiation (ALPN) extension
[TLS-ALPN].
HTTP/2 over TLS uses the “h2” protocol identifier. The “h2c”
protocol identifier MUST NOT be sent by a client or selected by a
server; the “h2c” protocol identifier describes a protocol that does
not use TLS.
Once TLS negotiation is complete, both the client and the server MUST
send a connection preface (Section 3.5).
3.4. Starting HTTP/2 with Prior Knowledge
A client can learn that a particular server supports HTTP/2 by other
means. For example, [ALT-SVC] describes a mechanism for advertising
this capability.
A client MUST send the connection preface (Section 3.5) and then MAY
immediately send HTTP/2 frames to such a server; servers can identify
these connections by the presence of the connection preface. This
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only affects the establishment of HTTP/2 connections over cleartext
TCP; implementations that support HTTP/2 over TLS MUST use protocol
negotiation in TLS [TLS-ALPN].
Likewise, the server MUST send a connection preface (Section 3.5).
Without additional information, prior support for HTTP/2 is not a
strong signal that a given server will support HTTP/2 for future
connections. For example, it is possible for server configurations
to change, for configurations to differ between instances in
clustered servers, or for network conditions to change.
3.5. HTTP/2 Connection Preface
In HTTP/2, each endpoint is required to send a connection preface as
a final confirmation of the protocol in use and to establish the
initial settings for the HTTP/2 connection. The client and server
each send a different connection preface.
The client connection preface starts with a sequence of 24 octets,
which in hex notation is:
0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
That is, the connection preface starts with the string “PRI *
HTTP/2.0\r\n\r\nSM\r\n\r\n”). This sequence MUST be followed by a
SETTINGS frame (Section 6.5), which MAY be empty. The client sends
the client connection preface immediately upon receipt of a 101
(Switching Protocols) response (indicating a successful upgrade) or
as the first application data octets of a TLS connection. If
starting an HTTP/2 connection with prior knowledge of server support
for the protocol, the client connection preface is sent upon
connection establishment.
Note: The client connection preface is selected so that a large
proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do
not attempt to process further frames. Note that this does not
address the concerns raised in [TALKING].
The server connection preface consists of a potentially empty
SETTINGS frame (Section 6.5) that MUST be the first frame the server
sends in the HTTP/2 connection.
The SETTINGS frames received from a peer as part of the connection
preface MUST be acknowledged (see Section 6.5.3) after sending the
connection preface.
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To avoid unnecessary latency, clients are permitted to send
additional frames to the server immediately after sending the client
connection preface, without waiting to receive the server connection
preface. It is important to note, however, that the server
connection preface SETTINGS frame might include parameters that
necessarily alter how a client is expected to communicate with the
server. Upon receiving the SETTINGS frame, the client is expected to
honor any parameters established. In some configurations, it is
possible for the server to transmit SETTINGS before the client sends
additional frames, providing an opportunity to avoid this issue.
Clients and servers MUST treat an invalid connection preface as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR. A GOAWAY
frame (Section 6.8) MAY be omitted in this case, since an invalid
preface indicates that the peer is not using HTTP/2.
4. HTTP Frames
Once the HTTP/2 connection is established, endpoints can begin
exchanging frames.
4.1. Frame Format
All frames begin with a fixed 9-octet header followed by a variable-
length payload.
+———————————————–+
| Length (24) |
+—————+—————+—————+
| Type (8) | Flags (8) |
+-+————-+—————+——————————-+
|R| Stream Identifier (31) |
+=+=============================================================+
| Frame Payload (0…) …
+—————————————————————+
Figure 1: Frame Layout
The fields of the frame header are defined as:
Length: The length of the frame payload expressed as an unsigned
24-bit integer. Values greater than 2^14 (16,384) MUST NOT be
sent unless the receiver has set a larger value for
SETTINGS_MAX_FRAME_SIZE.
The 9 octets of the frame header are not included in this value.
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Type: The 8-bit type of the frame. The frame type determines the
format and semantics of the frame. Implementations MUST ignore
and discard any frame that has a type that is unknown.
Flags: An 8-bit field reserved for boolean flags specific to the
frame type.
Flags are assigned semantics specific to the indicated frame type.
Flags that have no defined semantics for a particular frame type
MUST be ignored and MUST be left unset (0x0) when sending.
R: A reserved 1-bit field. The semantics of this bit are undefined,
and the bit MUST remain unset (0x0) when sending and MUST be
ignored when receiving.
Stream Identifier: A stream identifier (see Section 5.1.1) expressed
as an unsigned 31-bit integer. The value 0x0 is reserved for
frames that are associated with the connection as a whole as
opposed to an individual stream.
The structure and content of the frame payload is dependent entirely
on the frame type.
4.2. Frame Size
The size of a frame payload is limited by the maximum size that a
receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This
setting can have any value between 2^14 (16,384) and 2^24-1
(16,777,215) octets, inclusive.
All implementations MUST be capable of receiving and minimally
processing frames up to 2^14 octets in length, plus the 9-octet frame
header (Section 4.1). The size of the frame header is not included
when describing frame sizes.
Note: Certain frame types, such as PING (Section 6.7), impose
additional limits on the amount of payload data allowed.
An endpoint MUST send an error code of FRAME_SIZE_ERROR if a frame
exceeds the size defined in SETTINGS_MAX_FRAME_SIZE, exceeds any
limit defined for the frame type, or is too small to contain
mandatory frame data. A frame size error in a frame that could alter
the state of the entire connection MUST be treated as a connection
error (Section 5.4.1); this includes any frame carrying a header
block (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and
CONTINUATION), SETTINGS, and any frame with a stream identifier of 0.
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Endpoints are not obligated to use all available space in a frame.
Responsiveness can be improved by using frames that are smaller than
the permitted maximum size. Sending large frames can result in
delays in sending time-sensitive frames (such as RST_STREAM,
WINDOW_UPDATE, or PRIORITY), which, if blocked by the transmission of
a large frame, could affect performance.
4.3. Header Compression and Decompression
Just as in HTTP/1, a header field in HTTP/2 is a name with one or
more associated values. Header fields are used within HTTP request
and response messages as well as in server push operations (see
Section 8.2).
Header lists are collections of zero or more header fields. When
transmitted over a connection, a header list is serialized into a
header block using HTTP header compression [COMPRESSION]. The
serialized header block is then divided into one or more octet
sequences, called header block fragments, and transmitted within the
payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6), or
CONTINUATION (Section 6.10) frames.
The Cookie header field [COOKIE] is treated specially by the HTTP
mapping (see Section 8.1.2.5).
A receiving endpoint reassembles the header block by concatenating
its fragments and then decompresses the block to reconstruct the
header list.
A complete header block consists of either:
o a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag
set, or
o a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared
and one or more CONTINUATION frames, where the last CONTINUATION
frame has the END_HEADERS flag set.
Header compression is stateful. One compression context and one
decompression context are used for the entire connection. A decoding
error in a header block MUST be treated as a connection error
(Section 5.4.1) of type COMPRESSION_ERROR.
Each header block is processed as a discrete unit. Header blocks
MUST be transmitted as a contiguous sequence of frames, with no
interleaved frames of any other type or from any other stream. The
last frame in a sequence of HEADERS or CONTINUATION frames has the
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END_HEADERS flag set. The last frame in a sequence of PUSH_PROMISE
or CONTINUATION frames has the END_HEADERS flag set. This allows a
header block to be logically equivalent to a single frame.
Header block fragments can only be sent as the payload of HEADERS,
PUSH_PROMISE, or CONTINUATION frames because these frames carry data
that can modify the compression context maintained by a receiver. An
endpoint receiving HEADERS, PUSH_PROMISE, or CONTINUATION frames
needs to reassemble header blocks and perform decompression even if
the frames are to be discarded. A receiver MUST terminate the
connection with a connection error (Section 5.4.1) of type
COMPRESSION_ERROR if it does not decompress a header block.
5. Streams and Multiplexing
A “stream” is an independent, bidirectional sequence of frames
exchanged between the client and server within an HTTP/2 connection.
Streams have several important characteristics:
o A single HTTP/2 connection can contain multiple concurrently open
streams, with either endpoint interleaving frames from multiple
streams.
o Streams can be established and used unilaterally or shared by
either the client or server.
o Streams can be closed by either endpoint.
o The order in which frames are sent on a stream is significant.
Recipients process frames in the order they are received. In
particular, the order of HEADERS and DATA frames is semantically
significant.
o Streams are identified by an integer. Stream identifiers are
assigned to streams by the endpoint initiating the stream.
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5.1. Stream States
The lifecycle of a stream is shown in Figure 2.
+——–+
send PP | | recv PP
,——–| idle |——–.
/ | | \
v +——–+ v
+———-+ | +———-+
| | | send H / | |
,——| reserved | | recv H | reserved |——.
| | (local) | | | (remote) | |
| +———-+ v +———-+ |
| | +——–+ | |
| | recv ES | | send ES | |
| send H | ,——-| open |——-. | recv H |
| | / | | \ | |
| v v +——–+ v v |
| +———-+ | +———-+ |
| | half | | | half | |
| | closed | | send R / | closed | |
| | (remote) | | recv R | (local) | |
| +———-+ | +———-+ |
| | | | |
| | send ES / | recv ES / | |
| | send R / v send R / | |
| | recv R +——–+ recv R | |
| send R / ‘———–>| |<-----------’ send R / |
| recv R | closed | recv R |
‘----------------------->| |<----------------------’
+--------+
send: endpoint sends this frame
recv: endpoint receives this frame
H: HEADERS frame (with implied CONTINUATIONs)
PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
ES: END_STREAM flag
R: RST_STREAM frame
Figure 2: Stream States
Note that this diagram shows stream state transitions and the frames
and flags that affect those transitions only. In this regard,
CONTINUATION frames do not result in state transitions; they are
effectively part of the HEADERS or PUSH_PROMISE that they follow.
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For the purpose of state transitions, the END_STREAM flag is
processed as a separate event to the frame that bears it; a HEADERS
frame with the END_STREAM flag set can cause two state transitions.
Both endpoints have a subjective view of the state of a stream that
could be different when frames are in transit. Endpoints do not
coordinate the creation of streams; they are created unilaterally by
either endpoint. The negative consequences of a mismatch in states
are limited to the "closed" state after sending RST_STREAM, where
frames might be received for some time after closing.
Streams have the following states:
idle:
All streams start in the "idle" state.
The following transitions are valid from this state:
* Sending or receiving a HEADERS frame causes the stream to
become "open". The stream identifier is selected as described
in Section 5.1.1. The same HEADERS frame can also cause a
stream to immediately become "half-closed".
* Sending a PUSH_PROMISE frame on another stream reserves the
idle stream that is identified for later use. The stream state
for the reserved stream transitions to "reserved (local)".
* Receiving a PUSH_PROMISE frame on another stream reserves an
idle stream that is identified for later use. The stream state
for the reserved stream transitions to "reserved (remote)".
* Note that the PUSH_PROMISE frame is not sent on the idle stream
but references the newly reserved stream in the Promised Stream
ID field.
Receiving any frame other than HEADERS or PRIORITY on a stream in
this state MUST be treated as a connection error (Section 5.4.1)
of type PROTOCOL_ERROR.
reserved (local):
A stream in the "reserved (local)" state is one that has been
promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame
reserves an idle stream by associating the stream with an open
stream that was initiated by the remote peer (see Section 8.2).
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In this state, only the following transitions are possible:
* The endpoint can send a HEADERS frame. This causes the stream
to open in a "half-closed (remote)" state.
* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation.
An endpoint MUST NOT send any type of frame other than HEADERS,
RST_STREAM, or PRIORITY in this state.
A PRIORITY or WINDOW_UPDATE frame MAY be received in this state.
Receiving any type of frame other than RST_STREAM, PRIORITY, or
WINDOW_UPDATE on a stream in this state MUST be treated as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
reserved (remote):
A stream in the "reserved (remote)" state has been reserved by a
remote peer.
In this state, only the following transitions are possible:
* Receiving a HEADERS frame causes the stream to transition to
"half-closed (local)".
* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation.
An endpoint MAY send a PRIORITY frame in this state to
reprioritize the reserved stream. An endpoint MUST NOT send any
type of frame other than RST_STREAM, WINDOW_UPDATE, or PRIORITY in
this state.
Receiving any type of frame other than HEADERS, RST_STREAM, or
PRIORITY on a stream in this state MUST be treated as a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
open:
A stream in the "open" state may be used by both peers to send
frames of any type. In this state, sending peers observe
advertised stream-level flow-control limits (Section 5.2).
From this state, either endpoint can send a frame with an
END_STREAM flag set, which causes the stream to transition into
one of the "half-closed" states. An endpoint sending an
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END_STREAM flag causes the stream state to become "half-closed
(local)"; an endpoint receiving an END_STREAM flag causes the
stream state to become "half-closed (remote)".
Either endpoint can send a RST_STREAM frame from this state,
causing it to transition immediately to "closed".
half-closed (local):
A stream that is in the "half-closed (local)" state cannot be used
for sending frames other than WINDOW_UPDATE, PRIORITY, and
RST_STREAM.
A stream transitions from this state to "closed" when a frame that
contains an END_STREAM flag is received or when either peer sends
a RST_STREAM frame.
An endpoint can receive any type of frame in this state.
Providing flow-control credit using WINDOW_UPDATE frames is
necessary to continue receiving flow-controlled frames. In this
state, a receiver can ignore WINDOW_UPDATE frames, which might
arrive for a short period after a frame bearing the END_STREAM
flag is sent.
PRIORITY frames received in this state are used to reprioritize
streams that depend on the identified stream.
half-closed (remote):
A stream that is "half-closed (remote)" is no longer being used by
the peer to send frames. In this state, an endpoint is no longer
obligated to maintain a receiver flow-control window.
If an endpoint receives additional frames, other than
WINDOW_UPDATE, PRIORITY, or RST_STREAM, for a stream that is in
this state, it MUST respond with a stream error (Section 5.4.2) of
type STREAM_CLOSED.
A stream that is "half-closed (remote)" can be used by the
endpoint to send frames of any type. In this state, the endpoint
continues to observe advertised stream-level flow-control limits
(Section 5.2).
A stream can transition from this state to "closed" by sending a
frame that contains an END_STREAM flag or when either peer sends a
RST_STREAM frame.
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closed:
The "closed" state is the terminal state.
An endpoint MUST NOT send frames other than PRIORITY on a closed
stream. An endpoint that receives any frame other than PRIORITY
after receiving a RST_STREAM MUST treat that as a stream error
(Section 5.4.2) of type STREAM_CLOSED. Similarly, an endpoint
that receives any frames after receiving a frame with the
END_STREAM flag set MUST treat that as a connection error
(Section 5.4.1) of type STREAM_CLOSED, unless the frame is
permitted as described below.
WINDOW_UPDATE or RST_STREAM frames can be received in this state
for a short period after a DATA or HEADERS frame containing an
END_STREAM flag is sent. Until the remote peer receives and
processes RST_STREAM or the frame bearing the END_STREAM flag, it
might send frames of these types. Endpoints MUST ignore
WINDOW_UPDATE or RST_STREAM frames received in this state, though
endpoints MAY choose to treat frames that arrive a significant
time after sending END_STREAM as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
PRIORITY frames can be sent on closed streams to prioritize
streams that are dependent on the closed stream. Endpoints SHOULD
process PRIORITY frames, though they can be ignored if the stream
has been removed from the dependency tree (see Section 5.3.4).
If this state is reached as a result of sending a RST_STREAM
frame, the peer that receives the RST_STREAM might have already
sent -- or enqueued for sending -- frames on the stream that
cannot be withdrawn. An endpoint MUST ignore frames that it
receives on closed streams after it has sent a RST_STREAM frame.
An endpoint MAY choose to limit the period over which it ignores
frames and treat frames that arrive after this time as being in
error.
Flow-controlled frames (i.e., DATA) received after sending
RST_STREAM are counted toward the connection flow-control window.
Even though these frames might be ignored, because they are sent
before the sender receives the RST_STREAM, the sender will
consider the frames to count against the flow-control window.
An endpoint might receive a PUSH_PROMISE frame after it sends
RST_STREAM. PUSH_PROMISE causes a stream to become "reserved"
even if the associated stream has been reset. Therefore, a
RST_STREAM is needed to close an unwanted promised stream.
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In the absence of more specific guidance elsewhere in this document,
implementations SHOULD treat the receipt of a frame that is not
expressly permitted in the description of a state as a connection
error (Section 5.4.1) of type PROTOCOL_ERROR. Note that PRIORITY can
be sent and received in any stream state. Frames of unknown types
are ignored.
An example of the state transitions for an HTTP request/response
exchange can be found in Section 8.1. An example of the state
transitions for server push can be found in Sections 8.2.1 and 8.2.2.
5.1.1. Stream Identifiers
Streams are identified with an unsigned 31-bit integer. Streams
initiated by a client MUST use odd-numbered stream identifiers; those
initiated by the server MUST use even-numbered stream identifiers. A
stream identifier of zero (0x0) is used for connection control
messages; the stream identifier of zero cannot be used to establish a
new stream.
HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are
responded to with a stream identifier of one (0x1). After the
upgrade completes, stream 0x1 is "half-closed (local)" to the client.
Therefore, stream 0x1 cannot be selected as a new stream identifier
by a client that upgrades from HTTP/1.1.
The identifier of a newly established stream MUST be numerically
greater than all streams that the initiating endpoint has opened or
reserved. This governs streams that are opened using a HEADERS frame
and streams that are reserved using PUSH_PROMISE. An endpoint that
receives an unexpected stream identifier MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The first use of a new stream identifier implicitly closes all
streams in the "idle" state that might have been initiated by that
peer with a lower-valued stream identifier. For example, if a client
sends a HEADERS frame on stream 7 without ever sending a frame on
stream 5, then stream 5 transitions to the "closed" state when the
first frame for stream 7 is sent or received.
Stream identifiers cannot be reused. Long-lived connections can
result in an endpoint exhausting the available range of stream
identifiers. A client that is unable to establish a new stream
identifier can establish a new connection for new streams. A server
that is unable to establish a new stream identifier can send a GOAWAY
frame so that the client is forced to open a new connection for new
streams.
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5.1.2. Stream Concurrency
A peer can limit the number of concurrently active streams using the
SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within
a SETTINGS frame. The maximum concurrent streams setting is specific
to each endpoint and applies only to the peer that receives the
setting. That is, clients specify the maximum number of concurrent
streams the server can initiate, and servers specify the maximum
number of concurrent streams the client can initiate.
Streams that are in the "open" state or in either of the "half-
closed" states count toward the maximum number of streams that an
endpoint is permitted to open. Streams in any of these three states
count toward the limit advertised in the
SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the
"reserved" states do not count toward the stream limit.
Endpoints MUST NOT exceed the limit set by their peer. An endpoint
that receives a HEADERS frame that causes its advertised concurrent
stream limit to be exceeded MUST treat this as a stream error
(Section 5.4.2) of type PROTOCOL_ERROR or REFUSED_STREAM. The choice
of error code determines whether the endpoint wishes to enable
automatic retry (see Section 8.1.4) for details).
An endpoint that wishes to reduce the value of
SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current
number of open streams can either close streams that exceed the new
value or allow streams to complete.
5.2. Flow Control
Using streams for multiplexing introduces contention over use of the
TCP connection, resulting in blocked streams. A flow-control scheme
ensures that streams on the same connection do not destructively
interfere with each other. Flow control is used for both individual
streams and for the connection as a whole.
HTTP/2 provides for flow control through use of the WINDOW_UPDATE
frame (Section 6.9).
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5.2.1. Flow-Control Principles
HTTP/2 stream flow control aims to allow a variety of flow-control
algorithms to be used without requiring protocol changes. Flow
control in HTTP/2 has the following characteristics:
1. Flow control is specific to a connection. Both types of flow
control are between the endpoints of a single hop and not over
the entire end-to-end path.
2. Flow control is based on WINDOW_UPDATE frames. Receivers
advertise how many octets they are prepared to receive on a
stream and for the entire connection. This is a credit-based
scheme.
3. Flow control is directional with overall control provided by the
receiver. A receiver MAY choose to set any window size that it
desires for each stream and for the entire connection. A sender
MUST respect flow-control limits imposed by a receiver. Clients,
servers, and intermediaries all independently advertise their
flow-control window as a receiver and abide by the flow-control
limits set by their peer when sending.
4. The initial value for the flow-control window is 65,535 octets
for both new streams and the overall connection.
5. The frame type determines whether flow control applies to a
frame. Of the frames specified in this document, only DATA
frames are subject to flow control; all other frame types do not
consume space in the advertised flow-control window. This
ensures that important control frames are not blocked by flow
control.
6. Flow control cannot be disabled.
7. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE
frame (Section 6.9). This document does not stipulate how a
receiver decides when to send this frame or the value that it
sends, nor does it specify how a sender chooses to send packets.
Implementations are able to select any algorithm that suits their
needs.
Implementations are also responsible for managing how requests and
responses are sent based on priority, choosing how to avoid head-of-
line blocking for requests, and managing the creation of new streams.
Algorithm choices for these could interact with any flow-control
algorithm.
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5.2.2. Appropriate Use of Flow Control
Flow control is defined to protect endpoints that are operating under
resource constraints. For example, a proxy needs to share memory
between many connections and also might have a slow upstream
connection and a fast downstream one. Flow-control addresses cases
where the receiver is unable to process data on one stream yet wants
to continue to process other streams in the same connection.
Deployments that do not require this capability can advertise a flow-
control window of the maximum size (2^31-1) and can maintain this
window by sending a WINDOW_UPDATE frame when any data is received.
This effectively disables flow control for that receiver.
Conversely, a sender is always subject to the flow-control window
advertised by the receiver.
Deployments with constrained resources (for example, memory) can
employ flow control to limit the amount of memory a peer can consume.
Note, however, that this can lead to suboptimal use of available
network resources if flow control is enabled without knowledge of the
bandwidth-delay product (see [RFC7323]).
Even with full awareness of the current bandwidth-delay product,
implementation of flow control can be difficult. When using flow
control, the receiver MUST read from the TCP receive buffer in a
timely fashion. Failure to do so could lead to a deadlock when
critical frames, such as WINDOW_UPDATE, are not read and acted upon.
5.3. Stream Priority
A client can assign a priority for a new stream by including
prioritization information in the HEADERS frame (Section 6.2) that
opens the stream. At any other time, the PRIORITY frame
(Section 6.3) can be used to change the priority of a stream.
The purpose of prioritization is to allow an endpoint to express how
it would prefer its peer to allocate resources when managing
concurrent streams. Most importantly, priority can be used to select
streams for transmitting frames when there is limited capacity for
sending.
Streams can be prioritized by marking them as dependent on the
completion of other streams (Section 5.3.1). Each dependency is
assigned a relative weight, a number that is used to determine the
relative proportion of available resources that are assigned to
streams dependent on the same stream.
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Explicitly setting the priority for a stream is input to a
prioritization process. It does not guarantee any particular
processing or transmission order for the stream relative to any other
stream. An endpoint cannot force a peer to process concurrent
streams in a particular order using priority. Expressing priority is
therefore only a suggestion.
Prioritization information can be omitted from messages. Defaults
are used prior to any explicit values being provided (Section 5.3.5).
5.3.1. Stream Dependencies
Each stream can be given an explicit dependency on another stream.
Including a dependency expresses a preference to allocate resources
to the identified stream rather than to the dependent stream.
A stream that is not dependent on any other stream is given a stream
dependency of 0x0. In other words, the non-existent stream 0 forms
the root of the tree.
A stream that depends on another stream is a dependent stream. The
stream upon which a stream is dependent is a parent stream. A
dependency on a stream that is not currently in the tree -- such as a
stream in the "idle" state -- results in that stream being given a
default priority (Section 5.3.5).
When assigning a dependency on another stream, the stream is added as
a new dependency of the parent stream. Dependent streams that share
the same parent are not ordered with respect to each other. For
example, if streams B and C are dependent on stream A, and if stream
D is created with a dependency on stream A, this results in a
dependency order of A followed by B, C, and D in any order.
A A
/ \ ==> /|\
B C B D C
Figure 3: Example of Default Dependency Creation
An exclusive flag allows for the insertion of a new level of
dependencies. The exclusive flag causes the stream to become the
sole dependency of its parent stream, causing other dependencies to
become dependent on the exclusive stream. In the previous example,
if stream D is created with an exclusive dependency on stream A, this
results in D becoming the dependency parent of B and C.
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A
A |
/ \ ==> D
B C / \
B C
Figure 4: Example of Exclusive Dependency Creation
Inside the dependency tree, a dependent stream SHOULD only be
allocated resources if either all of the streams that it depends on
(the chain of parent streams up to 0x0) are closed or it is not
possible to make progress on them.
A stream cannot depend on itself. An endpoint MUST treat this as a
stream error (Section 5.4.2) of type PROTOCOL_ERROR.
5.3.2. Dependency Weighting
All dependent streams are allocated an integer weight between 1 and
256 (inclusive).
Streams with the same parent SHOULD be allocated resources
proportionally based on their weight. Thus, if stream B depends on
stream A with weight 4, stream C depends on stream A with weight 12,
and no progress can be made on stream A, stream B ideally receives
one-third of the resources allocated to stream C.
5.3.3. Reprioritization
Stream priorities are changed using the PRIORITY frame. Setting a
dependency causes a stream to become dependent on the identified
parent stream.
Dependent streams move with their parent stream if the parent is
reprioritized. Setting a dependency with the exclusive flag for a
reprioritized stream causes all the dependencies of the new parent
stream to become dependent on the reprioritized stream.
If a stream is made dependent on one of its own dependencies, the
formerly dependent stream is first moved to be dependent on the
reprioritized stream’s previous parent. The moved dependency retains
its weight.
For example, consider an original dependency tree where B and C
depend on A, D and E depend on C, and F depends on D. If A is made
dependent on D, then D takes the place of A. All other dependency
relationships stay the same, except for F, which becomes dependent on
A if the reprioritization is exclusive.
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x x x x
| / \ | |
A D A D D
/ \ / / \ / \ |
B C ==> F B C ==> F A OR A
/ \ | / \ /|\
D E E B C B C F
| | |
F E E
(intermediate) (non-exclusive) (exclusive)
Figure 5: Example of Dependency Reordering
5.3.4. Prioritization State Management
When a stream is removed from the dependency tree, its dependencies
can be moved to become dependent on the parent of the closed stream.
The weights of new dependencies are recalculated by distributing the
weight of the dependency of the closed stream proportionally based on
the weights of its dependencies.
Streams that are removed from the dependency tree cause some
prioritization information to be lost. Resources are shared between
streams with the same parent stream, which means that if a stream in
that set closes or becomes blocked, any spare capacity allocated to a
stream is distributed to the immediate neighbors of the stream.
However, if the common dependency is removed from the tree, those
streams share resources with streams at the next highest level.
For example, assume streams A and B share a parent, and streams C and
D both depend on stream A. Prior to the removal of stream A, if
streams A and D are unable to proceed, then stream C receives all the
resources dedicated to stream A. If stream A is removed from the
tree, the weight of stream A is divided between streams C and D. If
stream D is still unable to proceed, this results in stream C
receiving a reduced proportion of resources. For equal starting
weights, C receives one third, rather than one half, of available
resources.
It is possible for a stream to become closed while prioritization
information that creates a dependency on that stream is in transit.
If a stream identified in a dependency has no associated priority
information, then the dependent stream is instead assigned a default
priority (Section 5.3.5). This potentially creates suboptimal
prioritization, since the stream could be given a priority that is
different from what is intended.
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To avoid these problems, an endpoint SHOULD retain stream
prioritization state for a period after streams become closed. The
longer state is retained, the lower the chance that streams are
assigned incorrect or default priority values.
Similarly, streams that are in the “idle” state can be assigned
priority or become a parent of other streams. This allows for the
creation of a grouping node in the dependency tree, which enables
more flexible expressions of priority. Idle streams begin with a
default priority (Section 5.3.5).
The retention of priority information for streams that are not
counted toward the limit set by SETTINGS_MAX_CONCURRENT_STREAMS could
create a large state burden for an endpoint. Therefore, the amount
of prioritization state that is retained MAY be limited.
The amount of additional state an endpoint maintains for
prioritization could be dependent on load; under high load,
prioritization state can be discarded to limit resource commitments.
In extreme cases, an endpoint could even discard prioritization state
for active or reserved streams. If a limit is applied, endpoints
SHOULD maintain state for at least as many streams as allowed by
their setting for SETTINGS_MAX_CONCURRENT_STREAMS. Implementations
SHOULD also attempt to retain state for streams that are in active
use in the priority tree.
If it has retained enough state to do so, an endpoint receiving a
PRIORITY frame that changes the priority of a closed stream SHOULD
alter the dependencies of the streams that depend on it.
5.3.5. Default Priorities
All streams are initially assigned a non-exclusive dependency on
stream 0x0. Pushed streams (Section 8.2) initially depend on their
associated stream. In both cases, streams are assigned a default
weight of 16.
5.4. Error Handling
HTTP/2 framing permits two classes of error:
o An error condition that renders the entire connection unusable is
a connection error.
o An error in an individual stream is a stream error.
A list of error codes is included in Section 7.
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5.4.1. Connection Error Handling
A connection error is any error that prevents further processing of
the frame layer or corrupts any connection state.
An endpoint that encounters a connection error SHOULD first send a
GOAWAY frame (Section 6.8) with the stream identifier of the last
stream that it successfully received from its peer. The GOAWAY frame
includes an error code that indicates why the connection is
terminating. After sending the GOAWAY frame for an error condition,
the endpoint MUST close the TCP connection.
It is possible that the GOAWAY will not be reliably received by the
receiving endpoint ([RFC7230], Section 6.6 describes how an immediate
connection close can result in data loss). In the event of a
connection error, GOAWAY only provides a best-effort attempt to
communicate with the peer about why the connection is being
terminated.
An endpoint can end a connection at any time. In particular, an
endpoint MAY choose to treat a stream error as a connection error.
Endpoints SHOULD send a GOAWAY frame when ending a connection,
providing that circumstances permit it.
5.4.2. Stream Error Handling
A stream error is an error related to a specific stream that does not
affect processing of other streams.
An endpoint that detects a stream error sends a RST_STREAM frame
(Section 6.4) that contains the stream identifier of the stream where
the error occurred. The RST_STREAM frame includes an error code that
indicates the type of error.
A RST_STREAM is the last frame that an endpoint can send on a stream.
The peer that sends the RST_STREAM frame MUST be prepared to receive
any frames that were sent or enqueued for sending by the remote peer.
These frames can be ignored, except where they modify connection
state (such as the state maintained for header compression
(Section 4.3) or flow control).
Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame
for any stream. However, an endpoint MAY send additional RST_STREAM
frames if it receives frames on a closed stream after more than a
round-trip time. This behavior is permitted to deal with misbehaving
implementations.
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To avoid looping, an endpoint MUST NOT send a RST_STREAM in response
to a RST_STREAM frame.
5.4.3. Connection Termination
If the TCP connection is closed or reset while streams remain in
“open” or “half-closed” state, then the affected streams cannot be
automatically retried (see Section 8.1.4 for details).
5.5. Extending HTTP/2
HTTP/2 permits extension of the protocol. Within the limitations
described in this section, protocol extensions can be used to provide
additional services or alter any aspect of the protocol. Extensions
are effective only within the scope of a single HTTP/2 connection.
This applies to the protocol elements defined in this document. This
does not affect the existing options for extending HTTP, such as
defining new methods, status codes, or header fields.
Extensions are permitted to use new frame types (Section 4.1), new
settings (Section 6.5.2), or new error codes (Section 7). Registries
are established for managing these extension points: frame types
(Section 11.2), settings (Section 11.3), and error codes
(Section 11.4).
Implementations MUST ignore unknown or unsupported values in all
extensible protocol elements. Implementations MUST discard frames
that have unknown or unsupported types. This means that any of these
extension points can be safely used by extensions without prior
arrangement or negotiation. However, extension frames that appear in
the middle of a header block (Section 4.3) are not permitted; these
MUST be treated as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
Extensions that could change the semantics of existing protocol
components MUST be negotiated before being used. For example, an
extension that changes the layout of the HEADERS frame cannot be used
until the peer has given a positive signal that this is acceptable.
In this case, it could also be necessary to coordinate when the
revised layout comes into effect. Note that treating any frames
other than DATA frames as flow controlled is such a change in
semantics and can only be done through negotiation.
This document doesn’t mandate a specific method for negotiating the
use of an extension but notes that a setting (Section 6.5.2) could be
used for that purpose. If both peers set a value that indicates
willingness to use the extension, then the extension can be used. If
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a setting is used for extension negotiation, the initial value MUST
be defined in such a fashion that the extension is initially
disabled.
6. Frame Definitions
This specification defines a number of frame types, each identified
by a unique 8-bit type code. Each frame type serves a distinct
purpose in the establishment and management either of the connection
as a whole or of individual streams.
The transmission of specific frame types can alter the state of a
connection. If endpoints fail to maintain a synchronized view of the
connection state, successful communication within the connection will
no longer be possible. Therefore, it is important that endpoints
have a shared comprehension of how the state is affected by the use
any given frame.
6.1. DATA
DATA frames (type=0x0) convey arbitrary, variable-length sequences of
octets associated with a stream. One or more DATA frames are used,
for instance, to carry HTTP request or response payloads.
DATA frames MAY also contain padding. Padding can be added to DATA
frames to obscure the size of messages. Padding is a security
feature; see Section 10.7.
+—————+
|Pad Length? (8)|
+—————+———————————————–+
| Data (*) …
+—————————————————————+
| Padding (*) …
+—————————————————————+
Figure 6: DATA Frame Payload
The DATA frame contains the following fields:
Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is conditional (as
signified by a “?” in the diagram) and is only present if the
PADDED flag is set.
Data: Application data. The amount of data is the remainder of the
frame payload after subtracting the length of the other fields
that are present.
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Padding: Padding octets that contain no application semantic value.
Padding octets MUST be set to zero when sending. A receiver is
not obligated to verify padding but MAY treat non-zero padding as
a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The DATA frame defines the following flags:
END_STREAM (0x1): When set, bit 0 indicates that this frame is the
last that the endpoint will send for the identified stream.
Setting this flag causes the stream to enter one of the “half-
closed” states or the “closed” state (Section 5.1).
PADDED (0x8): When set, bit 3 indicates that the Pad Length field
and any padding that it describes are present.
DATA frames MUST be associated with a stream. If a DATA frame is
received whose stream identifier field is 0x0, the recipient MUST
respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
DATA frames are subject to flow control and can only be sent when a
stream is in the “open” or “half-closed (remote)” state. The entire
DATA frame payload is included in flow control, including the Pad
Length and Padding fields if present. If a DATA frame is received
whose stream is not in “open” or “half-closed (local)” state, the
recipient MUST respond with a stream error (Section 5.4.2) of type
STREAM_CLOSED.
The total number of padding octets is determined by the value of the
Pad Length field. If the length of the padding is the length of the
frame payload or greater, the recipient MUST treat this as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Note: A frame can be increased in size by one octet by including a
Pad Length field with a value of zero.
6.2. HEADERS
The HEADERS frame (type=0x1) is used to open a stream (Section 5.1),
and additionally carries a header block fragment. HEADERS frames can
be sent on a stream in the “idle”, “reserved (local)”, “open”, or
“half-closed (remote)” state.
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+—————+
|Pad Length? (8)|
+-+————-+———————————————–+
|E| Stream Dependency? (31) |
+-+————-+———————————————–+
| Weight? (8) |
+-+————-+———————————————–+
| Header Block Fragment (*) …
+—————————————————————+
| Padding (*) …
+—————————————————————+
Figure 7: HEADERS Frame Payload
The HEADERS frame payload has the following fields:
Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is only present if the
PADDED flag is set.
E: A single-bit flag indicating that the stream dependency is
exclusive (see Section 5.3). This field is only present if the
PRIORITY flag is set.
Stream Dependency: A 31-bit stream identifier for the stream that
this stream depends on (see Section 5.3). This field is only
present if the PRIORITY flag is set.
Weight: An unsigned 8-bit integer representing a priority weight for
the stream (see Section 5.3). Add one to the value to obtain a
weight between 1 and 256. This field is only present if the
PRIORITY flag is set.
Header Block Fragment: A header block fragment (Section 4.3).
Padding: Padding octets.
The HEADERS frame defines the following flags:
END_STREAM (0x1): When set, bit 0 indicates that the header block
(Section 4.3) is the last that the endpoint will send for the
identified stream.
A HEADERS frame carries the END_STREAM flag that signals the end
of a stream. However, a HEADERS frame with the END_STREAM flag
set can be followed by CONTINUATION frames on the same stream.
Logically, the CONTINUATION frames are part of the HEADERS frame.
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END_HEADERS (0x4): When set, bit 2 indicates that this frame
contains an entire header block (Section 4.3) and is not followed
by any CONTINUATION frames.
A HEADERS frame without the END_HEADERS flag set MUST be followed
by a CONTINUATION frame for the same stream. A receiver MUST
treat the receipt of any other type of frame or a frame on a
different stream as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
PADDED (0x8): When set, bit 3 indicates that the Pad Length field
and any padding that it describes are present.
PRIORITY (0x20): When set, bit 5 indicates that the Exclusive Flag
(E), Stream Dependency, and Weight fields are present; see
Section 5.3.
The payload of a HEADERS frame contains a header block fragment
(Section 4.3). A header block that does not fit within a HEADERS
frame is continued in a CONTINUATION frame (Section 6.10).
HEADERS frames MUST be associated with a stream. If a HEADERS frame
is received whose stream identifier field is 0x0, the recipient MUST
respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
The HEADERS frame changes the connection state as described in
Section 4.3.
The HEADERS frame can include padding. Padding fields and flags are
identical to those defined for DATA frames (Section 6.1). Padding
that exceeds the size remaining for the header block fragment MUST be
treated as a PROTOCOL_ERROR.
Prioritization information in a HEADERS frame is logically equivalent
to a separate PRIORITY frame, but inclusion in HEADERS avoids the
potential for churn in stream prioritization when new streams are
created. Prioritization fields in HEADERS frames subsequent to the
first on a stream reprioritize the stream (Section 5.3.3).
6.3. PRIORITY
The PRIORITY frame (type=0x2) specifies the sender-advised priority
of a stream (Section 5.3). It can be sent in any stream state,
including idle or closed streams.
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+-+————————————————————-+
|E| Stream Dependency (31) |
+-+————-+———————————————–+
| Weight (8) |
+-+————-+
Figure 8: PRIORITY Frame Payload
The payload of a PRIORITY frame contains the following fields:
E: A single-bit flag indicating that the stream dependency is
exclusive (see Section 5.3).
Stream Dependency: A 31-bit stream identifier for the stream that
this stream depends on (see Section 5.3).
Weight: An unsigned 8-bit integer representing a priority weight for
the stream (see Section 5.3). Add one to the value to obtain a
weight between 1 and 256.
The PRIORITY frame does not define any flags.
The PRIORITY frame always identifies a stream. If a PRIORITY frame
is received with a stream identifier of 0x0, the recipient MUST
respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
The PRIORITY frame can be sent on a stream in any state, though it
cannot be sent between consecutive frames that comprise a single
header block (Section 4.3). Note that this frame could arrive after
processing or frame sending has completed, which would cause it to
have no effect on the identified stream. For a stream that is in the
“half-closed (remote)” or “closed” state, this frame can only affect
processing of the identified stream and its dependent streams; it
does not affect frame transmission on that stream.
The PRIORITY frame can be sent for a stream in the “idle” or “closed”
state. This allows for the reprioritization of a group of dependent
streams by altering the priority of an unused or closed parent
stream.
A PRIORITY frame with a length other than 5 octets MUST be treated as
a stream error (Section 5.4.2) of type FRAME_SIZE_ERROR.
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6.4. RST_STREAM
The RST_STREAM frame (type=0x3) allows for immediate termination of a
stream. RST_STREAM is sent to request cancellation of a stream or to
indicate that an error condition has occurred.
+—————————————————————+
| Error Code (32) |
+—————————————————————+
Figure 9: RST_STREAM Frame Payload
The RST_STREAM frame contains a single unsigned, 32-bit integer
identifying the error code (Section 7). The error code indicates why
the stream is being terminated.
The RST_STREAM frame does not define any flags.
The RST_STREAM frame fully terminates the referenced stream and
causes it to enter the “closed” state. After receiving a RST_STREAM
on a stream, the receiver MUST NOT send additional frames for that
stream, with the exception of PRIORITY. However, after sending the
RST_STREAM, the sending endpoint MUST be prepared to receive and
process additional frames sent on the stream that might have been
sent by the peer prior to the arrival of the RST_STREAM.
RST_STREAM frames MUST be associated with a stream. If a RST_STREAM
frame is received with a stream identifier of 0x0, the recipient MUST
treat this as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
RST_STREAM frames MUST NOT be sent for a stream in the “idle” state.
If a RST_STREAM frame identifying an idle stream is received, the
recipient MUST treat this as a connection error (Section 5.4.1) of
type PROTOCOL_ERROR.
A RST_STREAM frame with a length other than 4 octets MUST be treated
as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.
6.5. SETTINGS
The SETTINGS frame (type=0x4) conveys configuration parameters that
affect how endpoints communicate, such as preferences and constraints
on peer behavior. The SETTINGS frame is also used to acknowledge the
receipt of those parameters. Individually, a SETTINGS parameter can
also be referred to as a “setting”.
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SETTINGS parameters are not negotiated; they describe characteristics
of the sending peer, which are used by the receiving peer. Different
values for the same parameter can be advertised by each peer. For
example, a client might set a high initial flow-control window,
whereas a server might set a lower value to conserve resources.
A SETTINGS frame MUST be sent by both endpoints at the start of a
connection and MAY be sent at any other time by either endpoint over
the lifetime of the connection. Implementations MUST support all of
the parameters defined by this specification.
Each parameter in a SETTINGS frame replaces any existing value for
that parameter. Parameters are processed in the order in which they
appear, and a receiver of a SETTINGS frame does not need to maintain
any state other than the current value of its parameters. Therefore,
the value of a SETTINGS parameter is the last value that is seen by a
receiver.
SETTINGS parameters are acknowledged by the receiving peer. To
enable this, the SETTINGS frame defines the following flag:
ACK (0x1): When set, bit 0 indicates that this frame acknowledges
receipt and application of the peer’s SETTINGS frame. When this
bit is set, the payload of the SETTINGS frame MUST be empty.
Receipt of a SETTINGS frame with the ACK flag set and a length
field value other than 0 MUST be treated as a connection error
(Section 5.4.1) of type FRAME_SIZE_ERROR. For more information,
see Section 6.5.3 (“Settings Synchronization”).
SETTINGS frames always apply to a connection, never a single stream.
The stream identifier for a SETTINGS frame MUST be zero (0x0). If an
endpoint receives a SETTINGS frame whose stream identifier field is
anything other than 0x0, the endpoint MUST respond with a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
The SETTINGS frame affects connection state. A badly formed or
incomplete SETTINGS frame MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
A SETTINGS frame with a length other than a multiple of 6 octets MUST
be treated as a connection error (Section 5.4.1) of type
FRAME_SIZE_ERROR.
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6.5.1. SETTINGS Format
The payload of a SETTINGS frame consists of zero or more parameters,
each consisting of an unsigned 16-bit setting identifier and an
unsigned 32-bit value.
+——————————-+
| Identifier (16) |
+——————————-+——————————-+
| Value (32) |
+—————————————————————+
Figure 10: Setting Format
6.5.2. Defined SETTINGS Parameters
The following parameters are defined:
SETTINGS_HEADER_TABLE_SIZE (0x1): Allows the sender to inform the
remote endpoint of the maximum size of the header compression
table used to decode header blocks, in octets. The encoder can
select any size equal to or less than this value by using
signaling specific to the header compression format inside a
header block (see [COMPRESSION]). The initial value is 4,096
octets.
SETTINGS_ENABLE_PUSH (0x2): This setting can be used to disable
server push (Section 8.2). An endpoint MUST NOT send a
PUSH_PROMISE frame if it receives this parameter set to a value of
0. An endpoint that has both set this parameter to 0 and had it
acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The initial value is 1, which indicates that server push is
permitted. Any value other than 0 or 1 MUST be treated as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
SETTINGS_MAX_CONCURRENT_STREAMS (0x3): Indicates the maximum number
of concurrent streams that the sender will allow. This limit is
directional: it applies to the number of streams that the sender
permits the receiver to create. Initially, there is no limit to
this value. It is recommended that this value be no smaller than
100, so as to not unnecessarily limit parallelism.
A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be
treated as special by endpoints. A zero value does prevent the
creation of new streams; however, this can also happen for any
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limit that is exhausted with active streams. Servers SHOULD only
set a zero value for short durations; if a server does not wish to
accept requests, closing the connection is more appropriate.
SETTINGS_INITIAL_WINDOW_SIZE (0x4): Indicates the sender’s initial
window size (in octets) for stream-level flow control. The
initial value is 2^16-1 (65,535) octets.
This setting affects the window size of all streams (see
Section 6.9.2).
Values above the maximum flow-control window size of 2^31-1 MUST
be treated as a connection error (Section 5.4.1) of type
FLOW_CONTROL_ERROR.
SETTINGS_MAX_FRAME_SIZE (0x5): Indicates the size of the largest
frame payload that the sender is willing to receive, in octets.
The initial value is 2^14 (16,384) octets. The value advertised
by an endpoint MUST be between this initial value and the maximum
allowed frame size (2^24-1 or 16,777,215 octets), inclusive.
Values outside this range MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
SETTINGS_MAX_HEADER_LIST_SIZE (0x6): This advisory setting informs a
peer of the maximum size of header list that the sender is
prepared to accept, in octets. The value is based on the
uncompressed size of header fields, including the length of the
name and value in octets plus an overhead of 32 octets for each
header field.
For any given request, a lower limit than what is advertised MAY
be enforced. The initial value of this setting is unlimited.
An endpoint that receives a SETTINGS frame with any unknown or
unsupported identifier MUST ignore that setting.
6.5.3. Settings Synchronization
Most values in SETTINGS benefit from or require an understanding of
when the peer has received and applied the changed parameter values.
In order to provide such synchronization timepoints, the recipient of
a SETTINGS frame in which the ACK flag is not set MUST apply the
updated parameters as soon as possible upon receipt.
The values in the SETTINGS frame MUST be processed in the order they
appear, with no other frame processing between values. Unsupported
parameters MUST be ignored. Once all values have been processed, the
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recipient MUST immediately emit a SETTINGS frame with the ACK flag
set. Upon receiving a SETTINGS frame with the ACK flag set, the
sender of the altered parameters can rely on the setting having been
applied.
If the sender of a SETTINGS frame does not receive an acknowledgement
within a reasonable amount of time, it MAY issue a connection error
(Section 5.4.1) of type SETTINGS_TIMEOUT.
6.6. PUSH_PROMISE
The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint
in advance of streams the sender intends to initiate. The
PUSH_PROMISE frame includes the unsigned 31-bit identifier of the
stream the endpoint plans to create along with a set of headers that
provide additional context for the stream. Section 8.2 contains a
thorough description of the use of PUSH_PROMISE frames.
+—————+
|Pad Length? (8)|
+-+————-+———————————————–+
|R| Promised Stream ID (31) |
+-+—————————–+——————————-+
| Header Block Fragment (*) …
+—————————————————————+
| Padding (*) …
+—————————————————————+
Figure 11: PUSH_PROMISE Payload Format
The PUSH_PROMISE frame payload has the following fields:
Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is only present if the
PADDED flag is set.
R: A single reserved bit.
Promised Stream ID: An unsigned 31-bit integer that identifies the
stream that is reserved by the PUSH_PROMISE. The promised stream
identifier MUST be a valid choice for the next stream sent by the
sender (see “new stream identifier” in Section 5.1.1).
Header Block Fragment: A header block fragment (Section 4.3)
containing request header fields.
Padding: Padding octets.
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The PUSH_PROMISE frame defines the following flags:
END_HEADERS (0x4): When set, bit 2 indicates that this frame
contains an entire header block (Section 4.3) and is not followed
by any CONTINUATION frames.
A PUSH_PROMISE frame without the END_HEADERS flag set MUST be
followed by a CONTINUATION frame for the same stream. A receiver
MUST treat the receipt of any other type of frame or a frame on a
different stream as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
PADDED (0x8): When set, bit 3 indicates that the Pad Length field
and any padding that it describes are present.
PUSH_PROMISE frames MUST only be sent on a peer-initiated stream that
is in either the “open” or “half-closed (remote)” state. The stream
identifier of a PUSH_PROMISE frame indicates the stream it is
associated with. If the stream identifier field specifies the value
0x0, a recipient MUST respond with a connection error (Section 5.4.1)
of type PROTOCOL_ERROR.
Promised streams are not required to be used in the order they are
promised. The PUSH_PROMISE only reserves stream identifiers for
later use.
PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of
the peer endpoint is set to 0. An endpoint that has set this setting
and has received acknowledgement MUST treat the receipt of a
PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
Recipients of PUSH_PROMISE frames can choose to reject promised
streams by returning a RST_STREAM referencing the promised stream
identifier back to the sender of the PUSH_PROMISE.
A PUSH_PROMISE frame modifies the connection state in two ways.
First, the inclusion of a header block (Section 4.3) potentially
modifies the state maintained for header compression. Second,
PUSH_PROMISE also reserves a stream for later use, causing the
promised stream to enter the “reserved” state. A sender MUST NOT
send a PUSH_PROMISE on a stream unless that stream is either “open”
or “half-closed (remote)”; the sender MUST ensure that the promised
stream is a valid choice for a new stream identifier (Section 5.1.1)
(that is, the promised stream MUST be in the “idle” state).
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Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame
causes the stream state to become indeterminate. A receiver MUST
treat the receipt of a PUSH_PROMISE on a stream that is neither
“open” nor “half-closed (local)” as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. However, an endpoint that
has sent RST_STREAM on the associated stream MUST handle PUSH_PROMISE
frames that might have been created before the RST_STREAM frame is
received and processed.
A receiver MUST treat the receipt of a PUSH_PROMISE that promises an
illegal stream identifier (Section 5.1.1) as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. Note that an illegal stream
identifier is an identifier for a stream that is not currently in the
“idle” state.
The PUSH_PROMISE frame can include padding. Padding fields and flags
are identical to those defined for DATA frames (Section 6.1).
6.7. PING
The PING frame (type=0x6) is a mechanism for measuring a minimal
round-trip time from the sender, as well as determining whether an
idle connection is still functional. PING frames can be sent from
any endpoint.
+—————————————————————+
| |
| Opaque Data (64) |
| |
+—————————————————————+
Figure 12: PING Payload Format
In addition to the frame header, PING frames MUST contain 8 octets of
opaque data in the payload. A sender can include any value it
chooses and use those octets in any fashion.
Receivers of a PING frame that does not include an ACK flag MUST send
a PING frame with the ACK flag set in response, with an identical
payload. PING responses SHOULD be given higher priority than any
other frame.
The PING frame defines the following flags:
ACK (0x1): When set, bit 0 indicates that this PING frame is a PING
response. An endpoint MUST set this flag in PING responses. An
endpoint MUST NOT respond to PING frames containing this flag.
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PING frames are not associated with any individual stream. If a PING
frame is received with a stream identifier field value other than
0x0, the recipient MUST respond with a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
Receipt of a PING frame with a length field value other than 8 MUST
be treated as a connection error (Section 5.4.1) of type
FRAME_SIZE_ERROR.
6.8. GOAWAY
The GOAWAY frame (type=0x7) is used to initiate shutdown of a
connection or to signal serious error conditions. GOAWAY allows an
endpoint to gracefully stop accepting new streams while still
finishing processing of previously established streams. This enables
administrative actions, like server maintenance.
There is an inherent race condition between an endpoint starting new
streams and the remote sending a GOAWAY frame. To deal with this
case, the GOAWAY contains the stream identifier of the last peer-
initiated stream that was or might be processed on the sending
endpoint in this connection. For instance, if the server sends a
GOAWAY frame, the identified stream is the highest-numbered stream
initiated by the client.
Once sent, the sender will ignore frames sent on streams initiated by
the receiver if the stream has an identifier higher than the included
last stream identifier. Receivers of a GOAWAY frame MUST NOT open
additional streams on the connection, although a new connection can
be established for new streams.
If the receiver of the GOAWAY has sent data on streams with a higher
stream identifier than what is indicated in the GOAWAY frame, those
streams are not or will not be processed. The receiver of the GOAWAY
frame can treat the streams as though they had never been created at
all, thereby allowing those streams to be retried later on a new
connection.
Endpoints SHOULD always send a GOAWAY frame before closing a
connection so that the remote peer can know whether a stream has been
partially processed or not. For example, if an HTTP client sends a
POST at the same time that a server closes a connection, the client
cannot know if the server started to process that POST request if the
server does not send a GOAWAY frame to indicate what streams it might
have acted on.
An endpoint might choose to close a connection without sending a
GOAWAY for misbehaving peers.
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A GOAWAY frame might not immediately precede closing of the
connection; a receiver of a GOAWAY that has no more use for the
connection SHOULD still send a GOAWAY frame before terminating the
connection.
+-+————————————————————-+
|R| Last-Stream-ID (31) |
+-+————————————————————-+
| Error Code (32) |
+—————————————————————+
| Additional Debug Data (*) |
+—————————————————————+
Figure 13: GOAWAY Payload Format
The GOAWAY frame does not define any flags.
The GOAWAY frame applies to the connection, not a specific stream.
An endpoint MUST treat a GOAWAY frame with a stream identifier other
than 0x0 as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
The last stream identifier in the GOAWAY frame contains the highest-
numbered stream identifier for which the sender of the GOAWAY frame
might have taken some action on or might yet take action on. All
streams up to and including the identified stream might have been
processed in some way. The last stream identifier can be set to 0 if
no streams were processed.
Note: In this context, “processed” means that some data from the
stream was passed to some higher layer of software that might have
taken some action as a result.
If a connection terminates without a GOAWAY frame, the last stream
identifier is effectively the highest possible stream identifier.
On streams with lower- or equal-numbered identifiers that were not
closed completely prior to the connection being closed, reattempting
requests, transactions, or any protocol activity is not possible,
with the exception of idempotent actions like HTTP GET, PUT, or
DELETE. Any protocol activity that uses higher-numbered streams can
be safely retried using a new connection.
Activity on streams numbered lower or equal to the last stream
identifier might still complete successfully. The sender of a GOAWAY
frame might gracefully shut down a connection by sending a GOAWAY
frame, maintaining the connection in an “open” state until all in-
progress streams complete.
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An endpoint MAY send multiple GOAWAY frames if circumstances change.
For instance, an endpoint that sends GOAWAY with NO_ERROR during
graceful shutdown could subsequently encounter a condition that
requires immediate termination of the connection. The last stream
identifier from the last GOAWAY frame received indicates which
streams could have been acted upon. Endpoints MUST NOT increase the
value they send in the last stream identifier, since the peers might
already have retried unprocessed requests on another connection.
A client that is unable to retry requests loses all requests that are
in flight when the server closes the connection. This is especially
true for intermediaries that might not be serving clients using
HTTP/2. A server that is attempting to gracefully shut down a
connection SHOULD send an initial GOAWAY frame with the last stream
identifier set to 2^31-1 and a NO_ERROR code. This signals to the
client that a shutdown is imminent and that initiating further
requests is prohibited. After allowing time for any in-flight stream
creation (at least one round-trip time), the server can send another
GOAWAY frame with an updated last stream identifier. This ensures
that a connection can be cleanly shut down without losing requests.
After sending a GOAWAY frame, the sender can discard frames for
streams initiated by the receiver with identifiers higher than the
identified last stream. However, any frames that alter connection
state cannot be completely ignored. For instance, HEADERS,
PUSH_PROMISE, and CONTINUATION frames MUST be minimally processed to
ensure the state maintained for header compression is consistent (see
Section 4.3); similarly, DATA frames MUST be counted toward the
connection flow-control window. Failure to process these frames can
cause flow control or header compression state to become
unsynchronized.
The GOAWAY frame also contains a 32-bit error code (Section 7) that
contains the reason for closing the connection.
Endpoints MAY append opaque data to the payload of any GOAWAY frame.
Additional debug data is intended for diagnostic purposes only and
carries no semantic value. Debug information could contain security-
or privacy-sensitive data. Logged or otherwise persistently stored
debug data MUST have adequate safeguards to prevent unauthorized
access.
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6.9. WINDOW_UPDATE
The WINDOW_UPDATE frame (type=0x8) is used to implement flow control;
see Section 5.2 for an overview.
Flow control operates at two levels: on each individual stream and on
the entire connection.
Both types of flow control are hop by hop, that is, only between the
two endpoints. Intermediaries do not forward WINDOW_UPDATE frames
between dependent connections. However, throttling of data transfer
by any receiver can indirectly cause the propagation of flow-control
information toward the original sender.
Flow control only applies to frames that are identified as being
subject to flow control. Of the frame types defined in this
document, this includes only DATA frames. Frames that are exempt
from flow control MUST be accepted and processed, unless the receiver
is unable to assign resources to handling the frame. A receiver MAY
respond with a stream error (Section 5.4.2) or connection error
(Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept
a frame.
+-+————————————————————-+
|R| Window Size Increment (31) |
+-+————————————————————-+
Figure 14: WINDOW_UPDATE Payload Format
The payload of a WINDOW_UPDATE frame is one reserved bit plus an
unsigned 31-bit integer indicating the number of octets that the
sender can transmit in addition to the existing flow-control window.
The legal range for the increment to the flow-control window is 1 to
2^31-1 (2,147,483,647) octets.
The WINDOW_UPDATE frame does not define any flags.
The WINDOW_UPDATE frame can be specific to a stream or to the entire
connection. In the former case, the frame’s stream identifier
indicates the affected stream; in the latter, the value “0” indicates
that the entire connection is the subject of the frame.
A receiver MUST treat the receipt of a WINDOW_UPDATE frame with an
flow-control window increment of 0 as a stream error (Section 5.4.2)
of type PROTOCOL_ERROR; errors on the connection flow-control window
MUST be treated as a connection error (Section 5.4.1).
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WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the
END_STREAM flag. This means that a receiver could receive a
WINDOW_UPDATE frame on a “half-closed (remote)” or “closed” stream.
A receiver MUST NOT treat this as an error (see Section 5.1).
A receiver that receives a flow-controlled frame MUST always account
for its contribution against the connection flow-control window,
unless the receiver treats this as a connection error
(Section 5.4.1). This is necessary even if the frame is in error.
The sender counts the frame toward the flow-control window, but if
the receiver does not, the flow-control window at the sender and
receiver can become different.
A WINDOW_UPDATE frame with a length other than 4 octets MUST be
treated as a connection error (Section 5.4.1) of type
FRAME_SIZE_ERROR.
6.9.1. The Flow-Control Window
Flow control in HTTP/2 is implemented using a window kept by each
sender on every stream. The flow-control window is a simple integer
value that indicates how many octets of data the sender is permitted
to transmit; as such, its size is a measure of the buffering capacity
of the receiver.
Two flow-control windows are applicable: the stream flow-control
window and the connection flow-control window. The sender MUST NOT
send a flow-controlled frame with a length that exceeds the space
available in either of the flow-control windows advertised by the
receiver. Frames with zero length with the END_STREAM flag set (that
is, an empty DATA frame) MAY be sent if there is no available space
in either flow-control window.
For flow-control calculations, the 9-octet frame header is not
counted.
After sending a flow-controlled frame, the sender reduces the space
available in both windows by the length of the transmitted frame.
The receiver of a frame sends a WINDOW_UPDATE frame as it consumes
data and frees up space in flow-control windows. Separate
WINDOW_UPDATE frames are sent for the stream- and connection-level
flow-control windows.
A sender that receives a WINDOW_UPDATE frame updates the
corresponding window by the amount specified in the frame.
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A sender MUST NOT allow a flow-control window to exceed 2^31-1
octets. If a sender receives a WINDOW_UPDATE that causes a flow-
control window to exceed this maximum, it MUST terminate either the
stream or the connection, as appropriate. For streams, the sender
sends a RST_STREAM with an error code of FLOW_CONTROL_ERROR; for the
connection, a GOAWAY frame with an error code of FLOW_CONTROL_ERROR
is sent.
Flow-controlled frames from the sender and WINDOW_UPDATE frames from
the receiver are completely asynchronous with respect to each other.
This property allows a receiver to aggressively update the window
size kept by the sender to prevent streams from stalling.
6.9.2. Initial Flow-Control Window Size
When an HTTP/2 connection is first established, new streams are
created with an initial flow-control window size of 65,535 octets.
The connection flow-control window is also 65,535 octets. Both
endpoints can adjust the initial window size for new streams by
including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS
frame that forms part of the connection preface. The connection
flow-control window can only be changed using WINDOW_UPDATE frames.
Prior to receiving a SETTINGS frame that sets a value for
SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default
initial window size when sending flow-controlled frames. Similarly,
the connection flow-control window is set to the default initial
window size until a WINDOW_UPDATE frame is received.
In addition to changing the flow-control window for streams that are
not yet active, a SETTINGS frame can alter the initial flow-control
window size for streams with active flow-control windows (that is,
streams in the “open” or “half-closed (remote)” state). When the
value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust
the size of all stream flow-control windows that it maintains by the
difference between the new value and the old value.
A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available
space in a flow-control window to become negative. A sender MUST
track the negative flow-control window and MUST NOT send new flow-
controlled frames until it receives WINDOW_UPDATE frames that cause
the flow-control window to become positive.
For example, if the client sends 60 KB immediately on connection
establishment and the server sets the initial window size to be 16
KB, the client will recalculate the available flow-control window to
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be -44 KB on receipt of the SETTINGS frame. The client retains a
negative flow-control window until WINDOW_UPDATE frames restore the
window to being positive, after which the client can resume sending.
A SETTINGS frame cannot alter the connection flow-control window.
An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that
causes any flow-control window to exceed the maximum size as a
connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.
6.9.3. Reducing the Stream Window Size
A receiver that wishes to use a smaller flow-control window than the
current size can send a new SETTINGS frame. However, the receiver
MUST be prepared to receive data that exceeds this window size, since
the sender might send data that exceeds the lower limit prior to
processing the SETTINGS frame.
After sending a SETTINGS frame that reduces the initial flow-control
window size, a receiver MAY continue to process streams that exceed
flow-control limits. Allowing streams to continue does not allow the
receiver to immediately reduce the space it reserves for flow-control
windows. Progress on these streams can also stall, since
WINDOW_UPDATE frames are needed to allow the sender to resume
sending. The receiver MAY instead send a RST_STREAM with an error
code of FLOW_CONTROL_ERROR for the affected streams.
6.10. CONTINUATION
The CONTINUATION frame (type=0x9) is used to continue a sequence of
header block fragments (Section 4.3). Any number of CONTINUATION
frames can be sent, as long as the preceding frame is on the same
stream and is a HEADERS, PUSH_PROMISE, or CONTINUATION frame without
the END_HEADERS flag set.
+—————————————————————+
| Header Block Fragment (*) …
+—————————————————————+
Figure 15: CONTINUATION Frame Payload
The CONTINUATION frame payload contains a header block fragment
(Section 4.3).
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The CONTINUATION frame defines the following flag:
END_HEADERS (0x4): When set, bit 2 indicates that this frame ends a
header block (Section 4.3).
If the END_HEADERS bit is not set, this frame MUST be followed by
another CONTINUATION frame. A receiver MUST treat the receipt of
any other type of frame or a frame on a different stream as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The CONTINUATION frame changes the connection state as defined in
Section 4.3.
CONTINUATION frames MUST be associated with a stream. If a
CONTINUATION frame is received whose stream identifier field is 0x0,
the recipient MUST respond with a connection error (Section 5.4.1) of
type PROTOCOL_ERROR.
A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or
CONTINUATION frame without the END_HEADERS flag set. A recipient
that observes violation of this rule MUST respond with a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
7. Error Codes
Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY
frames to convey the reasons for the stream or connection error.
Error codes share a common code space. Some error codes apply only
to either streams or the entire connection and have no defined
semantics in the other context.
The following error codes are defined:
NO_ERROR (0x0): The associated condition is not a result of an
error. For example, a GOAWAY might include this code to indicate
graceful shutdown of a connection.
PROTOCOL_ERROR (0x1): The endpoint detected an unspecific protocol
error. This error is for use when a more specific error code is
not available.
INTERNAL_ERROR (0x2): The endpoint encountered an unexpected
internal error.
FLOW_CONTROL_ERROR (0x3): The endpoint detected that its peer
violated the flow-control protocol.
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SETTINGS_TIMEOUT (0x4): The endpoint sent a SETTINGS frame but did
not receive a response in a timely manner. See Section 6.5.3
(“Settings Synchronization”).
STREAM_CLOSED (0x5): The endpoint received a frame after a stream
was half-closed.
FRAME_SIZE_ERROR (0x6): The endpoint received a frame with an
invalid size.
REFUSED_STREAM (0x7): The endpoint refused the stream prior to
performing any application processing (see Section 8.1.4 for
details).
CANCEL (0x8): Used by the endpoint to indicate that the stream is no
longer needed.
COMPRESSION_ERROR (0x9): The endpoint is unable to maintain the
header compression context for the connection.
CONNECT_ERROR (0xa): The connection established in response to a
CONNECT request (Section 8.3) was reset or abnormally closed.
ENHANCE_YOUR_CALM (0xb): The endpoint detected that its peer is
exhibiting a behavior that might be generating excessive load.
INADEQUATE_SECURITY (0xc): The underlying transport has properties
that do not meet minimum security requirements (see Section 9.2).
HTTP_1_1_REQUIRED (0xd): The endpoint requires that HTTP/1.1 be used
instead of HTTP/2.
Unknown or unsupported error codes MUST NOT trigger any special
behavior. These MAY be treated by an implementation as being
equivalent to INTERNAL_ERROR.
8. HTTP Message Exchanges
HTTP/2 is intended to be as compatible as possible with current uses
of HTTP. This means that, from the application perspective, the
features of the protocol are largely unchanged. To achieve this, all
request and response semantics are preserved, although the syntax of
conveying those semantics has changed.
Thus, the specification and requirements of HTTP/1.1 Semantics and
Content [RFC7231], Conditional Requests [RFC7232], Range Requests
[RFC7233], Caching [RFC7234], and Authentication [RFC7235] are
applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax
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and Routing [RFC7230], such as the HTTP and HTTPS URI schemes, are
also applicable in HTTP/2, but the expression of those semantics for
this protocol are defined in the sections below.
8.1. HTTP Request/Response Exchange
A client sends an HTTP request on a new stream, using a previously
unused stream identifier (Section 5.1.1). A server sends an HTTP
response on the same stream as the request.
An HTTP message (request or response) consists of:
1. for a response only, zero or more HEADERS frames (each followed
by zero or more CONTINUATION frames) containing the message
headers of informational (1xx) HTTP responses (see [RFC7230],
Section 3.2 and [RFC7231], Section 6.2),
2. one HEADERS frame (followed by zero or more CONTINUATION frames)
containing the message headers (see [RFC7230], Section 3.2),
3. zero or more DATA frames containing the payload body (see
[RFC7230], Section 3.3), and
4. optionally, one HEADERS frame, followed by zero or more
CONTINUATION frames containing the trailer-part, if present (see
[RFC7230], Section 4.1.2).
The last frame in the sequence bears an END_STREAM flag, noting that
a HEADERS frame bearing the END_STREAM flag can be followed by
CONTINUATION frames that carry any remaining portions of the header
block.
Other frames (from any stream) MUST NOT occur between the HEADERS
frame and any CONTINUATION frames that might follow.
HTTP/2 uses DATA frames to carry message payloads. The “chunked”
transfer encoding defined in Section 4.1 of [RFC7230] MUST NOT be
used in HTTP/2.
Trailing header fields are carried in a header block that also
terminates the stream. Such a header block is a sequence starting
with a HEADERS frame, followed by zero or more CONTINUATION frames,
where the HEADERS frame bears an END_STREAM flag. Header blocks
after the first that do not terminate the stream are not part of an
HTTP request or response.
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A HEADERS frame (and associated CONTINUATION frames) can only appear
at the start or end of a stream. An endpoint that receives a HEADERS
frame without the END_STREAM flag set after receiving a final (non-
informational) status code MUST treat the corresponding request or
response as malformed (Section 8.1.2.6).
An HTTP request/response exchange fully consumes a single stream. A
request starts with the HEADERS frame that puts the stream into an
“open” state. The request ends with a frame bearing END_STREAM,
which causes the stream to become “half-closed (local)” for the
client and “half-closed (remote)” for the server. A response starts
with a HEADERS frame and ends with a frame bearing END_STREAM, which
places the stream in the “closed” state.
An HTTP response is complete after the server sends — or the client
receives — a frame with the END_STREAM flag set (including any
CONTINUATION frames needed to complete a header block). A server can
send a complete response prior to the client sending an entire
request if the response does not depend on any portion of the request
that has not been sent and received. When this is true, a server MAY
request that the client abort transmission of a request without error
by sending a RST_STREAM with an error code of NO_ERROR after sending
a complete response (i.e., a frame with the END_STREAM flag).
Clients MUST NOT discard responses as a result of receiving such a
RST_STREAM, though clients can always discard responses at their
discretion for other reasons.
8.1.1. Upgrading from HTTP/2
HTTP/2 removes support for the 101 (Switching Protocols)
informational status code ([RFC7231], Section 6.2.2).
The semantics of 101 (Switching Protocols) aren’t applicable to a
multiplexed protocol. Alternative protocols are able to use the same
mechanisms that HTTP/2 uses to negotiate their use (see Section 3).
8.1.2. HTTP Header Fields
HTTP header fields carry information as a series of key-value pairs.
For a listing of registered HTTP headers, see the “Message Header
Field” registry maintained at
Just as in HTTP/1.x, header field names are strings of ASCII
characters that are compared in a case-insensitive fashion. However,
header field names MUST be converted to lowercase prior to their
encoding in HTTP/2. A request or response containing uppercase
header field names MUST be treated as malformed (Section 8.1.2.6).
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8.1.2.1. Pseudo-Header Fields
While HTTP/1.x used the message start-line (see [RFC7230],
Section 3.1) to convey the target URI, the method of the request, and
the status code for the response, HTTP/2 uses special pseudo-header
fields beginning with ’:’ character (ASCII 0x3a) for this purpose.
Pseudo-header fields are not HTTP header fields. Endpoints MUST NOT
generate pseudo-header fields other than those defined in this
document.
Pseudo-header fields are only valid in the context in which they are
defined. Pseudo-header fields defined for requests MUST NOT appear
in responses; pseudo-header fields defined for responses MUST NOT
appear in requests. Pseudo-header fields MUST NOT appear in
trailers. Endpoints MUST treat a request or response that contains
undefined or invalid pseudo-header fields as malformed
(Section 8.1.2.6).
All pseudo-header fields MUST appear in the header block before
regular header fields. Any request or response that contains a
pseudo-header field that appears in a header block after a regular
header field MUST be treated as malformed (Section 8.1.2.6).
8.1.2.2. Connection-Specific Header Fields
HTTP/2 does not use the Connection header field to indicate
connection-specific header fields; in this protocol, connection-
specific metadata is conveyed by other means. An endpoint MUST NOT
generate an HTTP/2 message containing connection-specific header
fields; any message containing connection-specific header fields MUST
be treated as malformed (Section 8.1.2.6).
The only exception to this is the TE header field, which MAY be
present in an HTTP/2 request; when it is, it MUST NOT contain any
value other than “trailers”.
This means that an intermediary transforming an HTTP/1.x message to
HTTP/2 will need to remove any header fields nominated by the
Connection header field, along with the Connection header field
itself. Such intermediaries SHOULD also remove other connection-
specific header fields, such as Keep-Alive, Proxy-Connection,
Transfer-Encoding, and Upgrade, even if they are not nominated by the
Connection header field.
Note: HTTP/2 purposefully does not support upgrade to another
protocol. The handshake methods described in Section 3 are
believed sufficient to negotiate the use of alternative protocols.
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8.1.2.3. Request Pseudo-Header Fields
The following pseudo-header fields are defined for HTTP/2 requests:
o The “:method” pseudo-header field includes the HTTP method
([RFC7231], Section 4).
o The “:scheme” pseudo-header field includes the scheme portion of
the target URI ([RFC3986], Section 3.1).
“:scheme” is not restricted to “http” and “https” schemed URIs. A
proxy or gateway can translate requests for non-HTTP schemes,
enabling the use of HTTP to interact with non-HTTP services.
o The “:authority” pseudo-header field includes the authority
portion of the target URI ([RFC3986], Section 3.2). The authority
MUST NOT include the deprecated “userinfo” subcomponent for “http”
or “https” schemed URIs.
To ensure that the HTTP/1.1 request line can be reproduced
accurately, this pseudo-header field MUST be omitted when
translating from an HTTP/1.1 request that has a request target in
origin or asterisk form (see [RFC7230], Section 5.3). Clients
that generate HTTP/2 requests directly SHOULD use the “:authority”
pseudo-header field instead of the Host header field. An
intermediary that converts an HTTP/2 request to HTTP/1.1 MUST
create a Host header field if one is not present in a request by
copying the value of the “:authority” pseudo-header field.
o The “:path” pseudo-header field includes the path and query parts
of the target URI (the “path-absolute” production and optionally a
’?’ character followed by the “query” production (see Sections 3.3
and 3.4 of [RFC3986]). A request in asterisk form includes the
value ’*’ for the “:path” pseudo-header field.
This pseudo-header field MUST NOT be empty for “http” or “https”
URIs; “http” or “https” URIs that do not contain a path component
MUST include a value of ’/’. The exception to this rule is an
OPTIONS request for an “http” or “https” URI that does not include
a path component; these MUST include a “:path” pseudo-header field
with a value of ’*’ (see [RFC7230], Section 5.3.4).
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All HTTP/2 requests MUST include exactly one valid value for the
“:method”, “:scheme”, and “:path” pseudo-header fields, unless it is
a CONNECT request (Section 8.3). An HTTP request that omits
mandatory pseudo-header fields is malformed (Section 8.1.2.6).
HTTP/2 does not define a way to carry the version identifier that is
included in the HTTP/1.1 request line.
8.1.2.4. Response Pseudo-Header Fields
For HTTP/2 responses, a single “:status” pseudo-header field is
defined that carries the HTTP status code field (see [RFC7231],
Section 6). This pseudo-header field MUST be included in all
responses; otherwise, the response is malformed (Section 8.1.2.6).
HTTP/2 does not define a way to carry the version or reason phrase
that is included in an HTTP/1.1 status line.
8.1.2.5. Compressing the Cookie Header Field
The Cookie header field [COOKIE] uses a semi-colon (“;”) to delimit
cookie-pairs (or “crumbs”). This header field doesn’t follow the
list construction rules in HTTP (see [RFC7230], Section 3.2.2), which
prevents cookie-pairs from being separated into different name-value
pairs. This can significantly reduce compression efficiency as
individual cookie-pairs are updated.
To allow for better compression efficiency, the Cookie header field
MAY be split into separate header fields, each with one or more
cookie-pairs. If there are multiple Cookie header fields after
decompression, these MUST be concatenated into a single octet string
using the two-octet delimiter of 0x3B, 0x20 (the ASCII string “; “)
before being passed into a non-HTTP/2 context, such as an HTTP/1.1
connection, or a generic HTTP server application.
Therefore, the following two lists of Cookie header fields are
semantically equivalent.
cookie: a=b; c=d; e=f
cookie: a=b
cookie: c=d
cookie: e=f
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8.1.2.6. Malformed Requests and Responses
A malformed request or response is one that is an otherwise valid
sequence of HTTP/2 frames but is invalid due to the presence of
extraneous frames, prohibited header fields, the absence of mandatory
header fields, or the inclusion of uppercase header field names.
A request or response that includes a payload body can include a
content-length header field. A request or response is also malformed
if the value of a content-length header field does not equal the sum
of the DATA frame payload lengths that form the body. A response
that is defined to have no payload, as described in [RFC7230],
Section 3.3.2, can have a non-zero content-length header field, even
though no content is included in DATA frames.
Intermediaries that process HTTP requests or responses (i.e., any
intermediary not acting as a tunnel) MUST NOT forward a malformed
request or response. Malformed requests or responses that are
detected MUST be treated as a stream error (Section 5.4.2) of type
PROTOCOL_ERROR.
For malformed requests, a server MAY send an HTTP response prior to
closing or resetting the stream. Clients MUST NOT accept a malformed
response. Note that these requirements are intended to protect
against several types of common attacks against HTTP; they are
deliberately strict because being permissive can expose
implementations to these vulnerabilities.
8.1.3. Examples
This section shows HTTP/1.1 requests and responses, with
illustrations of equivalent HTTP/2 requests and responses.
An HTTP GET request includes request header fields and no payload
body and is therefore transmitted as a single HEADERS frame, followed
by zero or more CONTINUATION frames containing the serialized block
of request header fields. The HEADERS frame in the following has
both the END_HEADERS and END_STREAM flags set; no CONTINUATION frames
are sent.
GET /resource HTTP/1.1 HEADERS
Host: example.org ==> + END_STREAM
Accept: image/jpeg + END_HEADERS
:method = GET
:scheme = https
:path = /resource
host = example.org
accept = image/jpeg
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Similarly, a response that includes only response header fields is
transmitted as a HEADERS frame (again, followed by zero or more
CONTINUATION frames) containing the serialized block of response
header fields.
HTTP/1.1 304 Not Modified HEADERS
ETag: “xyzzy” ==> + END_STREAM
Expires: Thu, 23 Jan … + END_HEADERS
:status = 304
etag = “xyzzy”
expires = Thu, 23 Jan …
An HTTP POST request that includes request header fields and payload
data is transmitted as one HEADERS frame, followed by zero or more
CONTINUATION frames containing the request header fields, followed by
one or more DATA frames, with the last CONTINUATION (or HEADERS)
frame having the END_HEADERS flag set and the final DATA frame having
the END_STREAM flag set:
POST /resource HTTP/1.1 HEADERS
Host: example.org ==> – END_STREAM
Content-Type: image/jpeg – END_HEADERS
Content-Length: 123 :method = POST
:path = /resource
{binary data} :scheme = https
CONTINUATION
+ END_HEADERS
content-type = image/jpeg
host = example.org
content-length = 123
DATA
+ END_STREAM
{binary data}
Note that data contributing to any given header field could be spread
between header block fragments. The allocation of header fields to
frames in this example is illustrative only.
A response that includes header fields and payload data is
transmitted as a HEADERS frame, followed by zero or more CONTINUATION
frames, followed by one or more DATA frames, with the last DATA frame
in the sequence having the END_STREAM flag set:
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HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> – END_STREAM
Content-Length: 123 + END_HEADERS
:status = 200
{binary data} content-type = image/jpeg
content-length = 123
DATA
+ END_STREAM
{binary data}
An informational response using a 1xx status code other than 101 is
transmitted as a HEADERS frame, followed by zero or more CONTINUATION
frames.
Trailing header fields are sent as a header block after both the
request or response header block and all the DATA frames have been
sent. The HEADERS frame starting the trailers header block has the
END_STREAM flag set.
The following example includes both a 100 (Continue) status code,
which is sent in response to a request containing a “100-continue”
token in the Expect header field, and trailing header fields:
HTTP/1.1 100 Continue HEADERS
Extension-Field: bar ==> – END_STREAM
+ END_HEADERS
:status = 100
extension-field = bar
HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> – END_STREAM
Transfer-Encoding: chunked + END_HEADERS
Trailer: Foo :status = 200
content-length = 123
123 content-type = image/jpeg
{binary data} trailer = Foo
0
Foo: bar DATA
– END_STREAM
{binary data}
HEADERS
+ END_STREAM
+ END_HEADERS
foo = bar
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8.1.4. Request Reliability Mechanisms in HTTP/2
In HTTP/1.1, an HTTP client is unable to retry a non-idempotent
request when an error occurs because there is no means to determine
the nature of the error. It is possible that some server processing
occurred prior to the error, which could result in undesirable
effects if the request were reattempted.
HTTP/2 provides two mechanisms for providing a guarantee to a client
that a request has not been processed:
o The GOAWAY frame indicates the highest stream number that might
have been processed. Requests on streams with higher numbers are
therefore guaranteed to be safe to retry.
o The REFUSED_STREAM error code can be included in a RST_STREAM
frame to indicate that the stream is being closed prior to any
processing having occurred. Any request that was sent on the
reset stream can be safely retried.
Requests that have not been processed have not failed; clients MAY
automatically retry them, even those with non-idempotent methods.
A server MUST NOT indicate that a stream has not been processed
unless it can guarantee that fact. If frames that are on a stream
are passed to the application layer for any stream, then
REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame
MUST include a stream identifier that is greater than or equal to the
given stream identifier.
In addition to these mechanisms, the PING frame provides a way for a
client to easily test a connection. Connections that remain idle can
become broken as some middleboxes (for instance, network address
translators or load balancers) silently discard connection bindings.
The PING frame allows a client to safely test whether a connection is
still active without sending a request.
8.2. Server Push
HTTP/2 allows a server to pre-emptively send (or “push”) responses
(along with corresponding “promised” requests) to a client in
association with a previous client-initiated request. This can be
useful when the server knows the client will need to have those
responses available in order to fully process the response to the
original request.
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A client can request that server push be disabled, though this is
negotiated for each hop independently. The SETTINGS_ENABLE_PUSH
setting can be set to 0 to indicate that server push is disabled.
Promised requests MUST be cacheable (see [RFC7231], Section 4.2.3),
MUST be safe (see [RFC7231], Section 4.2.1), and MUST NOT include a
request body. Clients that receive a promised request that is not
cacheable, that is not known to be safe, or that indicates the
presence of a request body MUST reset the promised stream with a
stream error (Section 5.4.2) of type PROTOCOL_ERROR. Note this could
result in the promised stream being reset if the client does not
recognize a newly defined method as being safe.
Pushed responses that are cacheable (see [RFC7234], Section 3) can be
stored by the client, if it implements an HTTP cache. Pushed
responses are considered successfully validated on the origin server
(e.g., if the “no-cache” cache response directive is present
([RFC7234], Section 5.2.2)) while the stream identified by the
promised stream ID is still open.
Pushed responses that are not cacheable MUST NOT be stored by any
HTTP cache. They MAY be made available to the application
separately.
The server MUST include a value in the “:authority” pseudo-header
field for which the server is authoritative (see Section 10.1). A
client MUST treat a PUSH_PROMISE for which the server is not
authoritative as a stream error (Section 5.4.2) of type
PROTOCOL_ERROR.
An intermediary can receive pushes from the server and choose not to
forward them on to the client. In other words, how to make use of
the pushed information is up to that intermediary. Equally, the
intermediary might choose to make additional pushes to the client,
without any action taken by the server.
A client cannot push. Thus, servers MUST treat the receipt of a
PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. Clients MUST reject any attempt to change the
SETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the
message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
8.2.1. Push Requests
Server push is semantically equivalent to a server responding to a
request; however, in this case, that request is also sent by the
server, as a PUSH_PROMISE frame.
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The PUSH_PROMISE frame includes a header block that contains a
complete set of request header fields that the server attributes to
the request. It is not possible to push a response to a request that
includes a request body.
Pushed responses are always associated with an explicit request from
the client. The PUSH_PROMISE frames sent by the server are sent on
that explicit request’s stream. The PUSH_PROMISE frame also includes
a promised stream identifier, chosen from the stream identifiers
available to the server (see Section 5.1.1).
The header fields in PUSH_PROMISE and any subsequent CONTINUATION
frames MUST be a valid and complete set of request header fields
(Section 8.1.2.3). The server MUST include a method in the “:method”
pseudo-header field that is safe and cacheable. If a client receives
a PUSH_PROMISE that does not include a complete and valid set of
header fields or the “:method” pseudo-header field identifies a
method that is not safe, it MUST respond with a stream error
(Section 5.4.2) of type PROTOCOL_ERROR.
The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to
sending any frames that reference the promised responses. This
avoids a race where clients issue requests prior to receiving any
PUSH_PROMISE frames.
For example, if the server receives a request for a document
containing embedded links to multiple image files and the server
chooses to push those additional images to the client, sending
PUSH_PROMISE frames before the DATA frames that contain the image
links ensures that the client is able to see that a resource will be
pushed before discovering embedded links. Similarly, if the server
pushes responses referenced by the header block (for instance, in
Link header fields), sending a PUSH_PROMISE before sending the header
block ensures that clients do not request those resources.
PUSH_PROMISE frames MUST NOT be sent by the client.
PUSH_PROMISE frames can be sent by the server in response to any
client-initiated stream, but the stream MUST be in either the “open”
or “half-closed (remote)” state with respect to the server.
PUSH_PROMISE frames are interspersed with the frames that comprise a
response, though they cannot be interspersed with HEADERS and
CONTINUATION frames that comprise a single header block.
Sending a PUSH_PROMISE frame creates a new stream and puts the stream
into the “reserved (local)” state for the server and the “reserved
(remote)” state for the client.
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8.2.2. Push Responses
After sending the PUSH_PROMISE frame, the server can begin delivering
the pushed response as a response (Section 8.1.2.4) on a server-
initiated stream that uses the promised stream identifier. The
server uses this stream to transmit an HTTP response, using the same
sequence of frames as defined in Section 8.1. This stream becomes
“half-closed” to the client (Section 5.1) after the initial HEADERS
frame is sent.
Once a client receives a PUSH_PROMISE frame and chooses to accept the
pushed response, the client SHOULD NOT issue any requests for the
promised response until after the promised stream has closed.
If the client determines, for any reason, that it does not wish to
receive the pushed response from the server or if the server takes
too long to begin sending the promised response, the client can send
a RST_STREAM frame, using either the CANCEL or REFUSED_STREAM code
and referencing the pushed stream’s identifier.
A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit
the number of responses that can be concurrently pushed by a server.
Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables
server push by preventing the server from creating the necessary
streams. This does not prohibit a server from sending PUSH_PROMISE
frames; clients need to reset any promised streams that are not
wanted.
Clients receiving a pushed response MUST validate that either the
server is authoritative (see Section 10.1) or the proxy that provided
the pushed response is configured for the corresponding request. For
example, a server that offers a certificate for only the
“example.com” DNS-ID or Common Name is not permitted to push a
response for “https://www.example.org/doc”.
The response for a PUSH_PROMISE stream begins with a HEADERS frame,
which immediately puts the stream into the “half-closed (remote)”
state for the server and “half-closed (local)” state for the client,
and ends with a frame bearing END_STREAM, which places the stream in
the “closed” state.
Note: The client never sends a frame with the END_STREAM flag for
a server push.
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8.3. The CONNECT Method
In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) is
used to convert an HTTP connection into a tunnel to a remote host.
CONNECT is primarily used with HTTP proxies to establish a TLS
session with an origin server for the purposes of interacting with
“https” resources.
In HTTP/2, the CONNECT method is used to establish a tunnel over a
single HTTP/2 stream to a remote host for similar purposes. The HTTP
header field mapping works as defined in Section 8.1.2.3 (“Request
Pseudo-Header Fields”), with a few differences. Specifically:
o The “:method” pseudo-header field is set to “CONNECT”.
o The “:scheme” and “:path” pseudo-header fields MUST be omitted.
o The “:authority” pseudo-header field contains the host and port to
connect to (equivalent to the authority-form of the request-target
of CONNECT requests (see [RFC7230], Section 5.3)).
A CONNECT request that does not conform to these restrictions is
malformed (Section 8.1.2.6).
A proxy that supports CONNECT establishes a TCP connection [TCP] to
the server identified in the “:authority” pseudo-header field. Once
this connection is successfully established, the proxy sends a
HEADERS frame containing a 2xx series status code to the client, as
defined in [RFC7231], Section 4.3.6.
After the initial HEADERS frame sent by each peer, all subsequent
DATA frames correspond to data sent on the TCP connection. The
payload of any DATA frames sent by the client is transmitted by the
proxy to the TCP server; data received from the TCP server is
assembled into DATA frames by the proxy. Frame types other than DATA
or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY)
MUST NOT be sent on a connected stream and MUST be treated as a
stream error (Section 5.4.2) if received.
The TCP connection can be closed by either peer. The END_STREAM flag
on a DATA frame is treated as being equivalent to the TCP FIN bit. A
client is expected to send a DATA frame with the END_STREAM flag set
after receiving a frame bearing the END_STREAM flag. A proxy that
receives a DATA frame with the END_STREAM flag set sends the attached
data with the FIN bit set on the last TCP segment. A proxy that
receives a TCP segment with the FIN bit set sends a DATA frame with
the END_STREAM flag set. Note that the final TCP segment or DATA
frame could be empty.
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A TCP connection error is signaled with RST_STREAM. A proxy treats
any error in the TCP connection, which includes receiving a TCP
segment with the RST bit set, as a stream error (Section 5.4.2) of
type CONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment
with the RST bit set if it detects an error with the stream or the
HTTP/2 connection.
9. Additional HTTP Requirements/Considerations
This section outlines attributes of the HTTP protocol that improve
interoperability, reduce exposure to known security vulnerabilities,
or reduce the potential for implementation variation.
9.1. Connection Management
HTTP/2 connections are persistent. For best performance, it is
expected that clients will not close connections until it is
determined that no further communication with a server is necessary
(for example, when a user navigates away from a particular web page)
or until the server closes the connection.
Clients SHOULD NOT open more than one HTTP/2 connection to a given
host and port pair, where the host is derived from a URI, a selected
alternative service [ALT-SVC], or a configured proxy.
A client can create additional connections as replacements, either to
replace connections that are near to exhausting the available stream
identifier space (Section 5.1.1), to refresh the keying material for
a TLS connection, or to replace connections that have encountered
errors (Section 5.4.1).
A client MAY open multiple connections to the same IP address and TCP
port using different Server Name Indication [TLS-EXT] values or to
provide different TLS client certificates but SHOULD avoid creating
multiple connections with the same configuration.
Servers are encouraged to maintain open connections for as long as
possible but are permitted to terminate idle connections if
necessary. When either endpoint chooses to close the transport-layer
TCP connection, the terminating endpoint SHOULD first send a GOAWAY
(Section 6.8) frame so that both endpoints can reliably determine
whether previously sent frames have been processed and gracefully
complete or terminate any necessary remaining tasks.
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9.1.1. Connection Reuse
Connections that are made to an origin server, either directly or
through a tunnel created using the CONNECT method (Section 8.3), MAY
be reused for requests with multiple different URI authority
components. A connection can be reused as long as the origin server
is authoritative (Section 10.1). For TCP connections without TLS,
this depends on the host having resolved to the same IP address.
For “https” resources, connection reuse additionally depends on
having a certificate that is valid for the host in the URI. The
certificate presented by the server MUST satisfy any checks that the
client would perform when forming a new TLS connection for the host
in the URI.
An origin server might offer a certificate with multiple
“subjectAltName” attributes or names with wildcards, one of which is
valid for the authority in the URI. For example, a certificate with
a “subjectAltName” of “*.example.com” might permit the use of the
same connection for requests to URIs starting with
“https://a.example.com/” and “https://b.example.com/”.
In some deployments, reusing a connection for multiple origins can
result in requests being directed to the wrong origin server. For
example, TLS termination might be performed by a middlebox that uses
the TLS Server Name Indication (SNI) [TLS-EXT] extension to select an
origin server. This means that it is possible for clients to send
confidential information to servers that might not be the intended
target for the request, even though the server is otherwise
authoritative.
A server that does not wish clients to reuse connections can indicate
that it is not authoritative for a request by sending a 421
(Misdirected Request) status code in response to the request (see
Section 9.1.2).
A client that is configured to use a proxy over HTTP/2 directs
requests to that proxy through a single connection. That is, all
requests sent via a proxy reuse the connection to the proxy.
9.1.2. The 421 (Misdirected Request) Status Code
The 421 (Misdirected Request) status code indicates that the request
was directed at a server that is not able to produce a response.
This can be sent by a server that is not configured to produce
responses for the combination of scheme and authority that are
included in the request URI.
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Clients receiving a 421 (Misdirected Request) response from a server
MAY retry the request — whether the request method is idempotent or
not — over a different connection. This is possible if a connection
is reused (Section 9.1.1) or if an alternative service is selected
[ALT-SVC].
This status code MUST NOT be generated by proxies.
A 421 response is cacheable by default, i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
9.2. Use of TLS Features
Implementations of HTTP/2 MUST use TLS version 1.2 [TLS12] or higher
for HTTP/2 over TLS. The general TLS usage guidance in [TLSBCP]
SHOULD be followed, with some additional restrictions that are
specific to HTTP/2.
The TLS implementation MUST support the Server Name Indication (SNI)
[TLS-EXT] extension to TLS. HTTP/2 clients MUST indicate the target
domain name when negotiating TLS.
Deployments of HTTP/2 that negotiate TLS 1.3 or higher need only
support and use the SNI extension; deployments of TLS 1.2 are subject
to the requirements in the following sections. Implementations are
encouraged to provide defaults that comply, but it is recognized that
deployments are ultimately responsible for compliance.
9.2.1. TLS 1.2 Features
This section describes restrictions on the TLS 1.2 feature set that
can be used with HTTP/2. Due to deployment limitations, it might not
be possible to fail TLS negotiation when these restrictions are not
met. An endpoint MAY immediately terminate an HTTP/2 connection that
does not meet these TLS requirements with a connection error
(Section 5.4.1) of type INADEQUATE_SECURITY.
A deployment of HTTP/2 over TLS 1.2 MUST disable compression. TLS
compression can lead to the exposure of information that would not
otherwise be revealed [RFC3749]. Generic compression is unnecessary
since HTTP/2 provides compression features that are more aware of
context and therefore likely to be more appropriate for use for
performance, security, or other reasons.
A deployment of HTTP/2 over TLS 1.2 MUST disable renegotiation. An
endpoint MUST treat a TLS renegotiation as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. Note that disabling
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renegotiation can result in long-lived connections becoming unusable
due to limits on the number of messages the underlying cipher suite
can encipher.
An endpoint MAY use renegotiation to provide confidentiality
protection for client credentials offered in the handshake, but any
renegotiation MUST occur prior to sending the connection preface. A
server SHOULD request a client certificate if it sees a renegotiation
request immediately after establishing a connection.
This effectively prevents the use of renegotiation in response to a
request for a specific protected resource. A future specification
might provide a way to support this use case. Alternatively, a
server might use an error (Section 5.4) of type HTTP_1_1_REQUIRED to
request the client use a protocol that supports renegotiation.
Implementations MUST support ephemeral key exchange sizes of at least
2048 bits for cipher suites that use ephemeral finite field Diffie-
Hellman (DHE) [TLS12] and 224 bits for cipher suites that use
ephemeral elliptic curve Diffie-Hellman (ECDHE) [RFC4492]. Clients
MUST accept DHE sizes of up to 4096 bits. Endpoints MAY treat
negotiation of key sizes smaller than the lower limits as a
connection error (Section 5.4.1) of type INADEQUATE_SECURITY.
9.2.2. TLS 1.2 Cipher Suites
A deployment of HTTP/2 over TLS 1.2 SHOULD NOT use any of the cipher
suites that are listed in the cipher suite black list (Appendix A).
Endpoints MAY choose to generate a connection error (Section 5.4.1)
of type INADEQUATE_SECURITY if one of the cipher suites from the
black list is negotiated. A deployment that chooses to use a black-
listed cipher suite risks triggering a connection error unless the
set of potential peers is known to accept that cipher suite.
Implementations MUST NOT generate this error in reaction to the
negotiation of a cipher suite that is not on the black list.
Consequently, when clients offer a cipher suite that is not on the
black list, they have to be prepared to use that cipher suite with
HTTP/2.
The black list includes the cipher suite that TLS 1.2 makes
mandatory, which means that TLS 1.2 deployments could have non-
intersecting sets of permitted cipher suites. To avoid this problem
causing TLS handshake failures, deployments of HTTP/2 that use TLS
1.2 MUST support TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [TLS-ECDHE]
with the P-256 elliptic curve [FIPS186].
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Note that clients might advertise support of cipher suites that are
on the black list in order to allow for connection to servers that do
not support HTTP/2. This allows servers to select HTTP/1.1 with a
cipher suite that is on the HTTP/2 black list. However, this can
result in HTTP/2 being negotiated with a black-listed cipher suite if
the application protocol and cipher suite are independently selected.
10. Security Considerations
10.1. Server Authority
HTTP/2 relies on the HTTP/1.1 definition of authority for determining
whether a server is authoritative in providing a given response (see
[RFC7230], Section 9.1). This relies on local name resolution for
the “http” URI scheme and the authenticated server identity for the
“https” scheme (see [RFC2818], Section 3).
10.2. Cross-Protocol Attacks
In a cross-protocol attack, an attacker causes a client to initiate a
transaction in one protocol toward a server that understands a
different protocol. An attacker might be able to cause the
transaction to appear as a valid transaction in the second protocol.
In combination with the capabilities of the web context, this can be
used to interact with poorly protected servers in private networks.
Completing a TLS handshake with an ALPN identifier for HTTP/2 can be
considered sufficient protection against cross-protocol attacks.
ALPN provides a positive indication that a server is willing to
proceed with HTTP/2, which prevents attacks on other TLS-based
protocols.
The encryption in TLS makes it difficult for attackers to control the
data that could be used in a cross-protocol attack on a cleartext
protocol.
The cleartext version of HTTP/2 has minimal protection against cross-
protocol attacks. The connection preface (Section 3.5) contains a
string that is designed to confuse HTTP/1.1 servers, but no special
protection is offered for other protocols. A server that is willing
to ignore parts of an HTTP/1.1 request containing an Upgrade header
field in addition to the client connection preface could be exposed
to a cross-protocol attack.
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10.3. Intermediary Encapsulation Attacks
The HTTP/2 header field encoding allows the expression of names that
are not valid field names in the Internet Message Syntax used by
HTTP/1.1. Requests or responses containing invalid header field
names MUST be treated as malformed (Section 8.1.2.6). An
intermediary therefore cannot translate an HTTP/2 request or response
containing an invalid field name into an HTTP/1.1 message.
Similarly, HTTP/2 allows header field values that are not valid.
While most of the values that can be encoded will not alter header
field parsing, carriage return (CR, ASCII 0xd), line feed (LF, ASCII
0xa), and the zero character (NUL, ASCII 0x0) might be exploited by
an attacker if they are translated verbatim. Any request or response
that contains a character not permitted in a header field value MUST
be treated as malformed (Section 8.1.2.6). Valid characters are
defined by the “field-content” ABNF rule in Section 3.2 of [RFC7230].
10.4. Cacheability of Pushed Responses
Pushed responses do not have an explicit request from the client; the
request is provided by the server in the PUSH_PROMISE frame.
Caching responses that are pushed is possible based on the guidance
provided by the origin server in the Cache-Control header field.
However, this can cause issues if a single server hosts more than one
tenant. For example, a server might offer multiple users each a
small portion of its URI space.
Where multiple tenants share space on the same server, that server
MUST ensure that tenants are not able to push representations of
resources that they do not have authority over. Failure to enforce
this would allow a tenant to provide a representation that would be
served out of cache, overriding the actual representation that the
authoritative tenant provides.
Pushed responses for which an origin server is not authoritative (see
Section 10.1) MUST NOT be used or cached.
10.5. Denial-of-Service Considerations
An HTTP/2 connection can demand a greater commitment of resources to
operate than an HTTP/1.1 connection. The use of header compression
and flow control depend on a commitment of resources for storing a
greater amount of state. Settings for these features ensure that
memory commitments for these features are strictly bounded.
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The number of PUSH_PROMISE frames is not constrained in the same
fashion. A client that accepts server push SHOULD limit the number
of streams it allows to be in the “reserved (remote)” state. An
excessive number of server push streams can be treated as a stream
error (Section 5.4.2) of type ENHANCE_YOUR_CALM.
Processing capacity cannot be guarded as effectively as state
capacity.
The SETTINGS frame can be abused to cause a peer to expend additional
processing time. This might be done by pointlessly changing SETTINGS
parameters, setting multiple undefined parameters, or changing the
same setting multiple times in the same frame. WINDOW_UPDATE or
PRIORITY frames can be abused to cause an unnecessary waste of
resources.
Large numbers of small or empty frames can be abused to cause a peer
to expend time processing frame headers. Note, however, that some
uses are entirely legitimate, such as the sending of an empty DATA or
CONTINUATION frame at the end of a stream.
Header compression also offers some opportunities to waste processing
resources; see Section 7 of [COMPRESSION] for more details on
potential abuses.
Limits in SETTINGS parameters cannot be reduced instantaneously,
which leaves an endpoint exposed to behavior from a peer that could
exceed the new limits. In particular, immediately after establishing
a connection, limits set by a server are not known to clients and
could be exceeded without being an obvious protocol violation.
All these features — i.e., SETTINGS changes, small frames, header
compression — have legitimate uses. These features become a burden
only when they are used unnecessarily or to excess.
An endpoint that doesn’t monitor this behavior exposes itself to a
risk of denial-of-service attack. Implementations SHOULD track the
use of these features and set limits on their use. An endpoint MAY
treat activity that is suspicious as a connection error
(Section 5.4.1) of type ENHANCE_YOUR_CALM.
10.5.1. Limits on Header Block Size
A large header block (Section 4.3) can cause an implementation to
commit a large amount of state. Header fields that are critical for
routing can appear toward the end of a header block, which prevents
streaming of header fields to their ultimate destination. This
ordering and other reasons, such as ensuring cache correctness, mean
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that an endpoint might need to buffer the entire header block. Since
there is no hard limit to the size of a header block, some endpoints
could be forced to commit a large amount of available memory for
header fields.
An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to advise peers
of limits that might apply on the size of header blocks. This
setting is only advisory, so endpoints MAY choose to send header
blocks that exceed this limit and risk having the request or response
being treated as malformed. This setting is specific to a
connection, so any request or response could encounter a hop with a
lower, unknown limit. An intermediary can attempt to avoid this
problem by passing on values presented by different peers, but they
are not obligated to do so.
A server that receives a larger header block than it is willing to
handle can send an HTTP 431 (Request Header Fields Too Large) status
code [RFC6585]. A client can discard responses that it cannot
process. The header block MUST be processed to ensure a consistent
connection state, unless the connection is closed.
10.5.2. CONNECT Issues
The CONNECT method can be used to create disproportionate load on an
proxy, since stream creation is relatively inexpensive when compared
to the creation and maintenance of a TCP connection. A proxy might
also maintain some resources for a TCP connection beyond the closing
of the stream that carries the CONNECT request, since the outgoing
TCP connection remains in the TIME_WAIT state. Therefore, a proxy
cannot rely on SETTINGS_MAX_CONCURRENT_STREAMS alone to limit the
resources consumed by CONNECT requests.
10.6. Use of Compression
Compression can allow an attacker to recover secret data when it is
compressed in the same context as data under attacker control.
HTTP/2 enables compression of header fields (Section 4.3); the
following concerns also apply to the use of HTTP compressed content-
codings ([RFC7231], Section 3.1.2.1).
There are demonstrable attacks on compression that exploit the
characteristics of the web (e.g., [BREACH]). The attacker induces
multiple requests containing varying plaintext, observing the length
of the resulting ciphertext in each, which reveals a shorter length
when a guess about the secret is correct.
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Implementations communicating on a secure channel MUST NOT compress
content that includes both confidential and attacker-controlled data
unless separate compression dictionaries are used for each source of
data. Compression MUST NOT be used if the source of data cannot be
reliably determined. Generic stream compression, such as that
provided by TLS, MUST NOT be used with HTTP/2 (see Section 9.2).
Further considerations regarding the compression of header fields are
described in [COMPRESSION].
10.7. Use of Padding
Padding within HTTP/2 is not intended as a replacement for general
purpose padding, such as might be provided by TLS [TLS12]. Redundant
padding could even be counterproductive. Correct application can
depend on having specific knowledge of the data that is being padded.
To mitigate attacks that rely on compression, disabling or limiting
compression might be preferable to padding as a countermeasure.
Padding can be used to obscure the exact size of frame content and is
provided to mitigate specific attacks within HTTP, for example,
attacks where compressed content includes both attacker-controlled
plaintext and secret data (e.g., [BREACH]).
Use of padding can result in less protection than might seem
immediately obvious. At best, padding only makes it more difficult
for an attacker to infer length information by increasing the number
of frames an attacker has to observe. Incorrectly implemented
padding schemes can be easily defeated. In particular, randomized
padding with a predictable distribution provides very little
protection; similarly, padding payloads to a fixed size exposes
information as payload sizes cross the fixed-sized boundary, which
could be possible if an attacker can control plaintext.
Intermediaries SHOULD retain padding for DATA frames but MAY drop
padding for HEADERS and PUSH_PROMISE frames. A valid reason for an
intermediary to change the amount of padding of frames is to improve
the protections that padding provides.
10.8. Privacy Considerations
Several characteristics of HTTP/2 provide an observer an opportunity
to correlate actions of a single client or server over time. These
include the value of settings, the manner in which flow-control
windows are managed, the way priorities are allocated to streams, the
timing of reactions to stimulus, and the handling of any features
that are controlled by settings.
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As far as these create observable differences in behavior, they could
be used as a basis for fingerprinting a specific client, as defined
in Section 1.8 of [HTML5].
HTTP/2’s preference for using a single TCP connection allows
correlation of a user’s activity on a site. Reusing connections for
different origins allows tracking across those origins.
Because the PING and SETTINGS frames solicit immediate responses,
they can be used by an endpoint to measure latency to their peer.
This might have privacy implications in certain scenarios.
11. IANA Considerations
A string for identifying HTTP/2 is entered into the “Application-
Layer Protocol Negotiation (ALPN) Protocol IDs” registry established
in [TLS-ALPN].
This document establishes a registry for frame types, settings, and
error codes. These new registries appear in the new “Hypertext
Transfer Protocol version 2 (HTTP/2) Parameters” section.
This document registers the HTTP2-Settings header field for use in
HTTP; it also registers the 421 (Misdirected Request) status code.
This document registers the “PRI” method for use in HTTP to avoid
collisions with the connection preface (Section 3.5).
11.1. Registration of HTTP/2 Identification Strings
This document creates two registrations for the identification of
HTTP/2 (see Section 3.3) in the “Application-Layer Protocol
Negotiation (ALPN) Protocol IDs” registry established in [TLS-ALPN].
The “h2” string identifies HTTP/2 when used over TLS:
Protocol: HTTP/2 over TLS
Identification Sequence: 0x68 0x32 (“h2”)
Specification: This document
The “h2c” string identifies HTTP/2 when used over cleartext TCP:
Protocol: HTTP/2 over TCP
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Identification Sequence: 0x68 0x32 0x63 (“h2c”)
Specification: This document
11.2. Frame Type Registry
This document establishes a registry for HTTP/2 frame type codes.
The “HTTP/2 Frame Type” registry manages an 8-bit space. The “HTTP/2
Frame Type” registry operates under either of the “IETF Review” or
“IESG Approval” policies [RFC5226] for values between 0x00 and 0xef,
with values between 0xf0 and 0xff being reserved for Experimental
Use.
New entries in this registry require the following information:
Frame Type: A name or label for the frame type.
Code: The 8-bit code assigned to the frame type.
Specification: A reference to a specification that includes a
description of the frame layout, its semantics, and flags that the
frame type uses, including any parts of the frame that are
conditionally present based on the value of flags.
The entries in the following table are registered by this document.
+—————+——+————–+
| Frame Type | Code | Section |
+—————+——+————–+
| DATA | 0x0 | Section 6.1 |
| HEADERS | 0x1 | Section 6.2 |
| PRIORITY | 0x2 | Section 6.3 |
| RST_STREAM | 0x3 | Section 6.4 |
| SETTINGS | 0x4 | Section 6.5 |
| PUSH_PROMISE | 0x5 | Section 6.6 |
| PING | 0x6 | Section 6.7 |
| GOAWAY | 0x7 | Section 6.8 |
| WINDOW_UPDATE | 0x8 | Section 6.9 |
| CONTINUATION | 0x9 | Section 6.10 |
+—————+——+————–+
11.3. Settings Registry
This document establishes a registry for HTTP/2 settings. The
“HTTP/2 Settings” registry manages a 16-bit space. The “HTTP/2
Settings” registry operates under the “Expert Review” policy
[RFC5226] for values in the range from 0x0000 to 0xefff, with values
between and 0xf000 and 0xffff being reserved for Experimental Use.
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New registrations are advised to provide the following information:
Name: A symbolic name for the setting. Specifying a setting name is
optional.
Code: The 16-bit code assigned to the setting.
Initial Value: An initial value for the setting.
Specification: An optional reference to a specification that
describes the use of the setting.
The entries in the following table are registered by this document.
+————————+——+—————+—————+
| Name | Code | Initial Value | Specification |
+————————+——+—————+—————+
| HEADER_TABLE_SIZE | 0x1 | 4096 | Section 6.5.2 |
| ENABLE_PUSH | 0x2 | 1 | Section 6.5.2 |
| MAX_CONCURRENT_STREAMS | 0x3 | (infinite) | Section 6.5.2 |
| INITIAL_WINDOW_SIZE | 0x4 | 65535 | Section 6.5.2 |
| MAX_FRAME_SIZE | 0x5 | 16384 | Section 6.5.2 |
| MAX_HEADER_LIST_SIZE | 0x6 | (infinite) | Section 6.5.2 |
+————————+——+—————+—————+
11.4. Error Code Registry
This document establishes a registry for HTTP/2 error codes. The
“HTTP/2 Error Code” registry manages a 32-bit space. The “HTTP/2
Error Code” registry operates under the “Expert Review” policy
[RFC5226].
Registrations for error codes are required to include a description
of the error code. An expert reviewer is advised to examine new
registrations for possible duplication with existing error codes.
Use of existing registrations is to be encouraged, but not mandated.
New registrations are advised to provide the following information:
Name: A name for the error code. Specifying an error code name is
optional.
Code: The 32-bit error code value.
Description: A brief description of the error code semantics, longer
if no detailed specification is provided.
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Specification: An optional reference for a specification that
defines the error code.
The entries in the following table are registered by this document.
+———————+——+———————-+—————+
| Name | Code | Description | Specification |
+———————+——+———————-+—————+
| NO_ERROR | 0x0 | Graceful shutdown | Section 7 |
| PROTOCOL_ERROR | 0x1 | Protocol error | Section 7 |
| | | detected | |
| INTERNAL_ERROR | 0x2 | Implementation fault | Section 7 |
| FLOW_CONTROL_ERROR | 0x3 | Flow-control limits | Section 7 |
| | | exceeded | |
| SETTINGS_TIMEOUT | 0x4 | Settings not | Section 7 |
| | | acknowledged | |
| STREAM_CLOSED | 0x5 | Frame received for | Section 7 |
| | | closed stream | |
| FRAME_SIZE_ERROR | 0x6 | Frame size incorrect | Section 7 |
| REFUSED_STREAM | 0x7 | Stream not processed | Section 7 |
| CANCEL | 0x8 | Stream cancelled | Section 7 |
| COMPRESSION_ERROR | 0x9 | Compression state | Section 7 |
| | | not updated | |
| CONNECT_ERROR | 0xa | TCP connection error | Section 7 |
| | | for CONNECT method | |
| ENHANCE_YOUR_CALM | 0xb | Processing capacity | Section 7 |
| | | exceeded | |
| INADEQUATE_SECURITY | 0xc | Negotiated TLS | Section 7 |
| | | parameters not | |
| | | acceptable | |
| HTTP_1_1_REQUIRED | 0xd | Use HTTP/1.1 for the | Section 7 |
| | | request | |
+———————+——+———————-+—————+
11.5. HTTP2-Settings Header Field Registration
This section registers the HTTP2-Settings header field in the
“Permanent Message Header Field Names” registry [BCP90].
Header field name: HTTP2-Settings
Applicable protocol: http
Status: standard
Author/Change controller: IETF
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Specification document(s): Section 3.2.1 of this document
Related information: This header field is only used by an HTTP/2
client for Upgrade-based negotiation.
11.6. PRI Method Registration
This section registers the “PRI” method in the “HTTP Method Registry”
([RFC7231], Section 8.1).
Method Name: PRI
Safe: Yes
Idempotent: Yes
Specification document(s): Section 3.5 of this document
Related information: This method is never used by an actual client.
This method will appear to be used when an HTTP/1.1 server or
intermediary attempts to parse an HTTP/2 connection preface.
11.7. The 421 (Misdirected Request) HTTP Status Code
This document registers the 421 (Misdirected Request) HTTP status
code in the “HTTP Status Codes” registry ([RFC7231], Section 8.2).
Status Code: 421
Short Description: Misdirected Request
Specification: Section 9.1.2 of this document
11.8. The h2c Upgrade Token
This document registers the “h2c” upgrade token in the “HTTP Upgrade
Tokens” registry ([RFC7230], Section 8.6).
Value: h2c
Description: Hypertext Transfer Protocol version 2 (HTTP/2)
Expected Version Tokens: None
Reference: Section 3.2 of this document
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RFC 7540 HTTP/2 May 2015
12. References
12.1. Normative References
[COMPRESSION] Peon, R. and H. Ruellan, “HPACK: Header Compression for
HTTP/2”, RFC 7541, DOI 10.17487/RFC7541, May 2015,
[COOKIE] Barth, A., “HTTP State Management Mechanism”, RFC 6265,
DOI 10.17487/RFC6265, April 2011,
[FIPS186] NIST, “Digital Signature Standard (DSS)”, FIPS PUB
186-4, July 2013,
[RFC2119] Bradner, S., “Key words for use in RFCs to Indicate
Requirement Levels”, BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
[RFC2818] Rescorla, E., “HTTP Over TLS”, RFC 2818, DOI 10.17487/
RFC2818, May 2000,
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
“Uniform Resource Identifier (URI): Generic Syntax”,
STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005,
[RFC4648] Josefsson, S., “The Base16, Base32, and Base64 Data
Encodings”, RFC 4648, DOI 10.17487/RFC4648, October
2006,
[RFC5226] Narten, T. and H. Alvestrand, “Guidelines for Writing
an IANA Considerations Section in RFCs”, BCP 26,
RFC 5226, DOI 10.17487/RFC5226, May 2008,
[RFC5234] Crocker, D., Ed. and P. Overell, “Augmented BNF for
Syntax Specifications: ABNF”, STD 68, RFC 5234,
DOI 10.17487/ RFC5234, January 2008,
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., “Hypertext
Transfer Protocol (HTTP/1.1): Message Syntax and
Routing”, RFC 7230, DOI 10.17487/RFC7230, June 2014,
Belshe, et al. Standards Track [Page 79]
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https://tools.ietf.org/pdf/rfc7541
http://www.rfc-editor.org/info/rfc7541
https://tools.ietf.org/pdf/rfc6265
http://www.rfc-editor.org/info/rfc6265
http://dx.doi.org/10.6028/NIST.FIPS.186-4
https://tools.ietf.org/pdf/bcp14
https://tools.ietf.org/pdf/rfc2119
https://tools.ietf.org/pdf/rfc2119
http://www.rfc-editor.org/info/rfc2119
https://tools.ietf.org/pdf/rfc2818
https://tools.ietf.org/pdf/rfc2818
http://www.rfc-editor.org/info/rfc2818
https://tools.ietf.org/pdf/rfc3986
http://www.rfc-editor.org/info/rfc3986
https://tools.ietf.org/pdf/rfc4648
http://www.rfc-editor.org/info/rfc4648
https://tools.ietf.org/pdf/bcp26
https://tools.ietf.org/pdf/rfc5226
http://www.rfc-editor.org/info/rfc5226
https://tools.ietf.org/pdf/rfc5234
https://tools.ietf.org/pdf/rfc5234
http://www.rfc-editor.org/info/rfc5234
https://tools.ietf.org/pdf/rfc7230
http://www.rfc-editor.org/info/rfc7230
RFC 7540 HTTP/2 May 2015
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., “Hypertext
Transfer Protocol (HTTP/1.1): Semantics and Content”,
RFC 7231, DOI 10.17487/RFC7231, June 2014,
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., “Hypertext
Transfer Protocol (HTTP/1.1): Conditional Requests”,
RFC 7232, DOI 10.17487/RFC7232, June 2014,
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
“Hypertext Transfer Protocol (HTTP/1.1): Range
Requests”, RFC 7233, DOI 10.17487/RFC7233, June 2014,
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”,
RFC 7234, DOI 10.17487/RFC7234, June 2014,
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., “Hypertext
Transfer Protocol (HTTP/1.1): Authentication”,
RFC 7235, DOI 10.17487/RFC7235, June 2014,
[TCP] Postel, J., “Transmission Control Protocol”, STD 7, RFC
793, DOI 10.17487/RFC0793, September 1981,
[TLS-ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
“Transport Layer Security (TLS) Application-Layer
Protocol Negotiation Extension”, RFC 7301,
DOI 10.17487/RFC7301, July 2014,
[TLS-ECDHE] Rescorla, E., “TLS Elliptic Curve Cipher Suites with
SHA-256/384 and AES Galois Counter Mode (GCM)”,
RFC 5289, DOI 10.17487/RFC5289, August 2008,
[TLS-EXT] Eastlake 3rd, D., “Transport Layer Security (TLS)
Extensions: Extension Definitions”, RFC 6066,
DOI 10.17487/RFC6066, January 2011,
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https://tools.ietf.org/pdf/rfc7232
http://www.rfc-editor.org/info/rfc7232
https://tools.ietf.org/pdf/rfc7233
http://www.rfc-editor.org/info/rfc7233
https://tools.ietf.org/pdf/rfc7234
http://www.rfc-editor.org/info/rfc7234
https://tools.ietf.org/pdf/rfc7235
http://www.rfc-editor.org/info/rfc7235
https://tools.ietf.org/pdf/rfc793
https://tools.ietf.org/pdf/rfc793
http://www.rfc-editor.org/info/rfc793
https://tools.ietf.org/pdf/rfc7301
http://www.rfc-editor.org/info/rfc7301
https://tools.ietf.org/pdf/rfc5289
http://www.rfc-editor.org/info/rfc5289
https://tools.ietf.org/pdf/rfc6066
http://www.rfc-editor.org/info/rfc6066
RFC 7540 HTTP/2 May 2015
[TLS12] Dierks, T. and E. Rescorla, “The Transport Layer
Security (TLS) Protocol Version 1.2”, RFC 5246,
DOI 10.17487/ RFC5246, August 2008,
12.2. Informative References
[ALT-SVC] Nottingham, M., McManus, P., and J. Reschke, “HTTP
Alternative Services”, Work in Progress, draft-ietf-
httpbis-alt-svc-06, February 2015.
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, “Registration
Procedures for Message Header Fields”, BCP 90,
RFC 3864, September 2004,
[BREACH] Gluck, Y., Harris, N., and A. Prado, “BREACH: Reviving
the CRIME Attack”, July 2013,
[HTML5] Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Doyle Navara, E., O’Connor, E., and S. Pfeiffer,
“HTML5”, W3C Recommendation REC-html5-20141028, October
2014,
[RFC3749] Hollenbeck, S., “Transport Layer Security Protocol
Compression Methods”, RFC 3749, DOI 10.17487/RFC3749,
May 2004,
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and
B. Moeller, “Elliptic Curve Cryptography (ECC) Cipher
Suites for Transport Layer Security (TLS)”, RFC 4492,
DOI 10.17487/RFC4492, May 2006,
[RFC6585] Nottingham, M. and R. Fielding, “Additional HTTP Status
Codes”, RFC 6585, DOI 10.17487/RFC6585, April 2012,
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., “TCP Extensions for High
Performance”, RFC 7323, DOI 10.17487/RFC7323, September
2014,
[TALKING] Huang, L., Chen, E., Barth, A., Rescorla, E., and C.
Jackson, “Talking to Yourself for Fun and Profit”,
2011,
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https://tools.ietf.org/pdf/draft-ietf-httpbis-alt-svc-06
https://tools.ietf.org/pdf/draft-ietf-httpbis-alt-svc-06
https://tools.ietf.org/pdf/bcp90
https://tools.ietf.org/pdf/rfc3864
http://www.rfc-editor.org/info/bcp90
http://breachattack.com/resources/
http://www.w3.org/TR/2014/REC-html5-20141028/
https://tools.ietf.org/pdf/rfc3749
http://www.rfc-editor.org/info/rfc3749
https://tools.ietf.org/pdf/rfc4492
http://www.rfc-editor.org/info/rfc4492
https://tools.ietf.org/pdf/rfc6585
http://www.rfc-editor.org/info/rfc6585
https://tools.ietf.org/pdf/rfc7323
http://www.rfc-editor.org/info/rfc7323
http://w2spconf.com/2011/papers/websocket.pdf
RFC 7540 HTTP/2 May 2015
[TLSBCP] Sheffer, Y., Holz, R., and P. Saint-Andre,
“Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)”, BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015,
Belshe, et al. Standards Track [Page 82]
https://tools.ietf.org/pdf/rfc7540
https://tools.ietf.org/pdf/bcp195
https://tools.ietf.org/pdf/rfc7525
http://www.rfc-editor.org/info/rfc7525
RFC 7540 HTTP/2 May 2015
Appendix A. TLS 1.2 Cipher Suite Black List
An HTTP/2 implementation MAY treat the negotiation of any of the
following cipher suites with TLS 1.2 as a connection error
(Section 5.4.1) of type INADEQUATE_SECURITY:
o TLS_NULL_WITH_NULL_NULL
o TLS_RSA_WITH_NULL_MD5
o TLS_RSA_WITH_NULL_SHA
o TLS_RSA_EXPORT_WITH_RC4_40_MD5
o TLS_RSA_WITH_RC4_128_MD5
o TLS_RSA_WITH_RC4_128_SHA
o TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
o TLS_RSA_WITH_IDEA_CBC_SHA
o TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
o TLS_RSA_WITH_DES_CBC_SHA
o TLS_RSA_WITH_3DES_EDE_CBC_SHA
o TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
o TLS_DH_DSS_WITH_DES_CBC_SHA
o TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA
o TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
o TLS_DH_RSA_WITH_DES_CBC_SHA
o TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA
o TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
o TLS_DHE_DSS_WITH_DES_CBC_SHA
o TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
o TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
Belshe, et al. Standards Track [Page 83]
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o TLS_DHE_RSA_WITH_DES_CBC_SHA
o TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA
o TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
o TLS_DH_anon_WITH_RC4_128_MD5
o TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
o TLS_DH_anon_WITH_DES_CBC_SHA
o TLS_DH_anon_WITH_3DES_EDE_CBC_SHA
o TLS_KRB5_WITH_DES_CBC_SHA
o TLS_KRB5_WITH_3DES_EDE_CBC_SHA
o TLS_KRB5_WITH_RC4_128_SHA
o TLS_KRB5_WITH_IDEA_CBC_SHA
o TLS_KRB5_WITH_DES_CBC_MD5
o TLS_KRB5_WITH_3DES_EDE_CBC_MD5
o TLS_KRB5_WITH_RC4_128_MD5
o TLS_KRB5_WITH_IDEA_CBC_MD5
o TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA
o TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA
o TLS_KRB5_EXPORT_WITH_RC4_40_SHA
o TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5
o TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5
o TLS_KRB5_EXPORT_WITH_RC4_40_MD5
o TLS_PSK_WITH_NULL_SHA
o TLS_DHE_PSK_WITH_NULL_SHA
o TLS_RSA_PSK_WITH_NULL_SHA
Belshe, et al. Standards Track [Page 84]
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o TLS_RSA_WITH_AES_128_CBC_SHA
o TLS_DH_DSS_WITH_AES_128_CBC_SHA
o TLS_DH_RSA_WITH_AES_128_CBC_SHA
o TLS_DHE_DSS_WITH_AES_128_CBC_SHA
o TLS_DHE_RSA_WITH_AES_128_CBC_SHA
o TLS_DH_anon_WITH_AES_128_CBC_SHA
o TLS_RSA_WITH_AES_256_CBC_SHA
o TLS_DH_DSS_WITH_AES_256_CBC_SHA
o TLS_DH_RSA_WITH_AES_256_CBC_SHA
o TLS_DHE_DSS_WITH_AES_256_CBC_SHA
o TLS_DHE_RSA_WITH_AES_256_CBC_SHA
o TLS_DH_anon_WITH_AES_256_CBC_SHA
o TLS_RSA_WITH_NULL_SHA256
o TLS_RSA_WITH_AES_128_CBC_SHA256
o TLS_RSA_WITH_AES_256_CBC_SHA256
o TLS_DH_DSS_WITH_AES_128_CBC_SHA256
o TLS_DH_RSA_WITH_AES_128_CBC_SHA256
o TLS_DHE_DSS_WITH_AES_128_CBC_SHA256
o TLS_RSA_WITH_CAMELLIA_128_CBC_SHA
o TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA
o TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA
o TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA
o TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA
o TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA
Belshe, et al. Standards Track [Page 85]
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o TLS_DHE_RSA_WITH_AES_128_CBC_SHA256
o TLS_DH_DSS_WITH_AES_256_CBC_SHA256
o TLS_DH_RSA_WITH_AES_256_CBC_SHA256
o TLS_DHE_DSS_WITH_AES_256_CBC_SHA256
o TLS_DHE_RSA_WITH_AES_256_CBC_SHA256
o TLS_DH_anon_WITH_AES_128_CBC_SHA256
o TLS_DH_anon_WITH_AES_256_CBC_SHA256
o TLS_RSA_WITH_CAMELLIA_256_CBC_SHA
o TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA
o TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA
o TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA
o TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA
o TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA
o TLS_PSK_WITH_RC4_128_SHA
o TLS_PSK_WITH_3DES_EDE_CBC_SHA
o TLS_PSK_WITH_AES_128_CBC_SHA
o TLS_PSK_WITH_AES_256_CBC_SHA
o TLS_DHE_PSK_WITH_RC4_128_SHA
o TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA
o TLS_DHE_PSK_WITH_AES_128_CBC_SHA
o TLS_DHE_PSK_WITH_AES_256_CBC_SHA
o TLS_RSA_PSK_WITH_RC4_128_SHA
o TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA
o TLS_RSA_PSK_WITH_AES_128_CBC_SHA
Belshe, et al. Standards Track [Page 86]
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RFC 7540 HTTP/2 May 2015
o TLS_RSA_PSK_WITH_AES_256_CBC_SHA
o TLS_RSA_WITH_SEED_CBC_SHA
o TLS_DH_DSS_WITH_SEED_CBC_SHA
o TLS_DH_RSA_WITH_SEED_CBC_SHA
o TLS_DHE_DSS_WITH_SEED_CBC_SHA
o TLS_DHE_RSA_WITH_SEED_CBC_SHA
o TLS_DH_anon_WITH_SEED_CBC_SHA
o TLS_RSA_WITH_AES_128_GCM_SHA256
o TLS_RSA_WITH_AES_256_GCM_SHA384
o TLS_DH_RSA_WITH_AES_128_GCM_SHA256
o TLS_DH_RSA_WITH_AES_256_GCM_SHA384
o TLS_DH_DSS_WITH_AES_128_GCM_SHA256
o TLS_DH_DSS_WITH_AES_256_GCM_SHA384
o TLS_DH_anon_WITH_AES_128_GCM_SHA256
o TLS_DH_anon_WITH_AES_256_GCM_SHA384
o TLS_PSK_WITH_AES_128_GCM_SHA256
o TLS_PSK_WITH_AES_256_GCM_SHA384
o TLS_RSA_PSK_WITH_AES_128_GCM_SHA256
o TLS_RSA_PSK_WITH_AES_256_GCM_SHA384
o TLS_PSK_WITH_AES_128_CBC_SHA256
o TLS_PSK_WITH_AES_256_CBC_SHA384
o TLS_PSK_WITH_NULL_SHA256
o TLS_PSK_WITH_NULL_SHA384
o TLS_DHE_PSK_WITH_AES_128_CBC_SHA256
Belshe, et al. Standards Track [Page 87]
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RFC 7540 HTTP/2 May 2015
o TLS_DHE_PSK_WITH_AES_256_CBC_SHA384
o TLS_DHE_PSK_WITH_NULL_SHA256
o TLS_DHE_PSK_WITH_NULL_SHA384
o TLS_RSA_PSK_WITH_AES_128_CBC_SHA256
o TLS_RSA_PSK_WITH_AES_256_CBC_SHA384
o TLS_RSA_PSK_WITH_NULL_SHA256
o TLS_RSA_PSK_WITH_NULL_SHA384
o TLS_RSA_WITH_CAMELLIA_128_CBC_SHA256
o TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA256
o TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA256
o TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA256
o TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA256
o TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA256
o TLS_RSA_WITH_CAMELLIA_256_CBC_SHA256
o TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA256
o TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA256
o TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA256
o TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA256
o TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA256
o TLS_EMPTY_RENEGOTIATION_INFO_SCSV
o TLS_ECDH_ECDSA_WITH_NULL_SHA
o TLS_ECDH_ECDSA_WITH_RC4_128_SHA
o TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA
o TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA
Belshe, et al. Standards Track [Page 88]
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RFC 7540 HTTP/2 May 2015
o TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA
o TLS_ECDHE_ECDSA_WITH_NULL_SHA
o TLS_ECDHE_ECDSA_WITH_RC4_128_SHA
o TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA
o TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
o TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
o TLS_ECDH_RSA_WITH_NULL_SHA
o TLS_ECDH_RSA_WITH_RC4_128_SHA
o TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA
o TLS_ECDH_RSA_WITH_AES_128_CBC_SHA
o TLS_ECDH_RSA_WITH_AES_256_CBC_SHA
o TLS_ECDHE_RSA_WITH_NULL_SHA
o TLS_ECDHE_RSA_WITH_RC4_128_SHA
o TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
o TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
o TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA
o TLS_ECDH_anon_WITH_NULL_SHA
o TLS_ECDH_anon_WITH_RC4_128_SHA
o TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA
o TLS_ECDH_anon_WITH_AES_128_CBC_SHA
o TLS_ECDH_anon_WITH_AES_256_CBC_SHA
o TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA
o TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA
o TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA
Belshe, et al. Standards Track [Page 89]
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RFC 7540 HTTP/2 May 2015
o TLS_SRP_SHA_WITH_AES_128_CBC_SHA
o TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA
o TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA
o TLS_SRP_SHA_WITH_AES_256_CBC_SHA
o TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA
o TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA
o TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
o TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384
o TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256
o TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384
o TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
o TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
o TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256
o TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384
o TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256
o TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384
o TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256
o TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384
o TLS_ECDHE_PSK_WITH_RC4_128_SHA
o TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA
o TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA
o TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA
o TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256
o TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384
Belshe, et al. Standards Track [Page 90]
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RFC 7540 HTTP/2 May 2015
o TLS_ECDHE_PSK_WITH_NULL_SHA
o TLS_ECDHE_PSK_WITH_NULL_SHA256
o TLS_ECDHE_PSK_WITH_NULL_SHA384
o TLS_RSA_WITH_ARIA_128_CBC_SHA256
o TLS_RSA_WITH_ARIA_256_CBC_SHA384
o TLS_DH_DSS_WITH_ARIA_128_CBC_SHA256
o TLS_DH_DSS_WITH_ARIA_256_CBC_SHA384
o TLS_DH_RSA_WITH_ARIA_128_CBC_SHA256
o TLS_DH_RSA_WITH_ARIA_256_CBC_SHA384
o TLS_DHE_DSS_WITH_ARIA_128_CBC_SHA256
o TLS_DHE_DSS_WITH_ARIA_256_CBC_SHA384
o TLS_DHE_RSA_WITH_ARIA_128_CBC_SHA256
o TLS_DHE_RSA_WITH_ARIA_256_CBC_SHA384
o TLS_DH_anon_WITH_ARIA_128_CBC_SHA256
o TLS_DH_anon_WITH_ARIA_256_CBC_SHA384
o TLS_ECDHE_ECDSA_WITH_ARIA_128_CBC_SHA256
o TLS_ECDHE_ECDSA_WITH_ARIA_256_CBC_SHA384
o TLS_ECDH_ECDSA_WITH_ARIA_128_CBC_SHA256
o TLS_ECDH_ECDSA_WITH_ARIA_256_CBC_SHA384
o TLS_ECDHE_RSA_WITH_ARIA_128_CBC_SHA256
o TLS_ECDHE_RSA_WITH_ARIA_256_CBC_SHA384
o TLS_ECDH_RSA_WITH_ARIA_128_CBC_SHA256
o TLS_ECDH_RSA_WITH_ARIA_256_CBC_SHA384
o TLS_RSA_WITH_ARIA_128_GCM_SHA256
Belshe, et al. Standards Track [Page 91]
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o TLS_RSA_WITH_ARIA_256_GCM_SHA384
o TLS_DH_RSA_WITH_ARIA_128_GCM_SHA256
o TLS_DH_RSA_WITH_ARIA_256_GCM_SHA384
o TLS_DH_DSS_WITH_ARIA_128_GCM_SHA256
o TLS_DH_DSS_WITH_ARIA_256_GCM_SHA384
o TLS_DH_anon_WITH_ARIA_128_GCM_SHA256
o TLS_DH_anon_WITH_ARIA_256_GCM_SHA384
o TLS_ECDH_ECDSA_WITH_ARIA_128_GCM_SHA256
o TLS_ECDH_ECDSA_WITH_ARIA_256_GCM_SHA384
o TLS_ECDH_RSA_WITH_ARIA_128_GCM_SHA256
o TLS_ECDH_RSA_WITH_ARIA_256_GCM_SHA384
o TLS_PSK_WITH_ARIA_128_CBC_SHA256
o TLS_PSK_WITH_ARIA_256_CBC_SHA384
o TLS_DHE_PSK_WITH_ARIA_128_CBC_SHA256
o TLS_DHE_PSK_WITH_ARIA_256_CBC_SHA384
o TLS_RSA_PSK_WITH_ARIA_128_CBC_SHA256
o TLS_RSA_PSK_WITH_ARIA_256_CBC_SHA384
o TLS_PSK_WITH_ARIA_128_GCM_SHA256
o TLS_PSK_WITH_ARIA_256_GCM_SHA384
o TLS_RSA_PSK_WITH_ARIA_128_GCM_SHA256
o TLS_RSA_PSK_WITH_ARIA_256_GCM_SHA384
o TLS_ECDHE_PSK_WITH_ARIA_128_CBC_SHA256
o TLS_ECDHE_PSK_WITH_ARIA_256_CBC_SHA384
o TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_CBC_SHA256
Belshe, et al. Standards Track [Page 92]
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RFC 7540 HTTP/2 May 2015
o TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_CBC_SHA384
o TLS_ECDH_ECDSA_WITH_CAMELLIA_128_CBC_SHA256
o TLS_ECDH_ECDSA_WITH_CAMELLIA_256_CBC_SHA384
o TLS_ECDHE_RSA_WITH_CAMELLIA_128_CBC_SHA256
o TLS_ECDHE_RSA_WITH_CAMELLIA_256_CBC_SHA384
o TLS_ECDH_RSA_WITH_CAMELLIA_128_CBC_SHA256
o TLS_ECDH_RSA_WITH_CAMELLIA_256_CBC_SHA384
o TLS_RSA_WITH_CAMELLIA_128_GCM_SHA256
o TLS_RSA_WITH_CAMELLIA_256_GCM_SHA384
o TLS_DH_RSA_WITH_CAMELLIA_128_GCM_SHA256
o TLS_DH_RSA_WITH_CAMELLIA_256_GCM_SHA384
o TLS_DH_DSS_WITH_CAMELLIA_128_GCM_SHA256
o TLS_DH_DSS_WITH_CAMELLIA_256_GCM_SHA384
o TLS_DH_anon_WITH_CAMELLIA_128_GCM_SHA256
o TLS_DH_anon_WITH_CAMELLIA_256_GCM_SHA384
o TLS_ECDH_ECDSA_WITH_CAMELLIA_128_GCM_SHA256
o TLS_ECDH_ECDSA_WITH_CAMELLIA_256_GCM_SHA384
o TLS_ECDH_RSA_WITH_CAMELLIA_128_GCM_SHA256
o TLS_ECDH_RSA_WITH_CAMELLIA_256_GCM_SHA384
o TLS_PSK_WITH_CAMELLIA_128_GCM_SHA256
o TLS_PSK_WITH_CAMELLIA_256_GCM_SHA384
o TLS_RSA_PSK_WITH_CAMELLIA_128_GCM_SHA256
o TLS_RSA_PSK_WITH_CAMELLIA_256_GCM_SHA384
o TLS_PSK_WITH_CAMELLIA_128_CBC_SHA256
Belshe, et al. Standards Track [Page 93]
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o TLS_PSK_WITH_CAMELLIA_256_CBC_SHA384
o TLS_DHE_PSK_WITH_CAMELLIA_128_CBC_SHA256
o TLS_DHE_PSK_WITH_CAMELLIA_256_CBC_SHA384
o TLS_RSA_PSK_WITH_CAMELLIA_128_CBC_SHA256
o TLS_RSA_PSK_WITH_CAMELLIA_256_CBC_SHA384
o TLS_ECDHE_PSK_WITH_CAMELLIA_128_CBC_SHA256
o TLS_ECDHE_PSK_WITH_CAMELLIA_256_CBC_SHA384
o TLS_RSA_WITH_AES_128_CCM
o TLS_RSA_WITH_AES_256_CCM
o TLS_RSA_WITH_AES_128_CCM_8
o TLS_RSA_WITH_AES_256_CCM_8
o TLS_PSK_WITH_AES_128_CCM
o TLS_PSK_WITH_AES_256_CCM
o TLS_PSK_WITH_AES_128_CCM_8
o TLS_PSK_WITH_AES_256_CCM_8
Note: This list was assembled from the set of registered TLS
cipher suites at the time of writing. This list includes those
cipher suites that do not offer an ephemeral key exchange and
those that are based on the TLS null, stream, or block cipher type
(as defined in Section 6.2.3 of [TLS12]). Additional cipher
suites with these properties could be defined; these would not be
explicitly prohibited.
Belshe, et al. Standards Track [Page 94]
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RFC 7540 HTTP/2 May 2015
Acknowledgements
This document includes substantial input from the following
individuals:
o Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa
Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam
Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay,
Paul Amer, Fan Yang, and Jonathan Leighton (SPDY contributors).
o Gabriel Montenegro and Willy Tarreau (Upgrade mechanism).
o William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro,
Jitu Padhye, Roberto Peon, and Rob Trace (Flow control).
o Mike Bishop (Extensibility).
o Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike
Bishop, and Herve Ruellan (Substantial editorial contributions).
o Kari Hurtta, Tatsuhiro Tsujikawa, Greg Wilkins, Poul-Henning Kamp,
and Jonathan Thackray.
o Alexey Melnikov, who was an editor of this document in 2013.
A substantial proportion of Martin’s contribution was supported by
Microsoft during his employment there.
The Japanese HTTP/2 community provided invaluable contributions,
including a number of implementations as well as numerous technical
and editorial contributions.
Belshe, et al. Standards Track [Page 95]
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RFC 7540 HTTP/2 May 2015
Authors’ Addresses
Mike Belshe
BitGo
EMail: mike@belshe.com
Roberto Peon
Google, Inc
EMail: fenix@google.com
Martin Thomson (editor)
Mozilla
331 E Evelyn Street
Mountain View, CA 94041
United States
EMail: martin.thomson@gmail.com
Belshe, et al. Standards Track [Page 96]
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